Complete initial copy edit of itm.md

Copy edit file for active voice, American English and precise language.

Author: Amanda Butler

docs/reference/contributing/target/itm.md

@@ -1,26 +1,26 @@
 ### Instrumented Trace Macrocell
 
-For targets with Arm CoreSight (e.g. Cortex-M3 and Cortex-M4) the Instrumented Trace Macrocell provides a lightweight, non-intrusive way for collecting debug trace output. 
+For targets with Arm CoreSight (for example, Cortex-M3 and Cortex-M4), the Instrumented Trace Macrocell provides a lightweight, nonintrusive way to collect debug trace output. 
 
 #### Assumptions
 
-* The target supports Arm CoreSight.
-* The target has SWO connected either to a compatible interface chip or exposed as a debug pin.
+- The target supports Arm CoreSight.
+- The target has SWO connected either to a compatible interface chip or exposed as a debug pin.
 
 ##### Defined behavior
 
-* Targets must implement the function `void itm_init(void)` and add `ITM` to the `device_has` section in `target.json`.
-* When `void itm_init(void)` is called, the debug clock for the ITM must be initialized and the SWO pin configured for debug output.
-* `void itm_init(void)` should only have to modify the clock pre-scaler in the generic register `TPI->ACPR`. The generic ITM registers will be initialized by the helper function `mbed_itm_init`. 
-* `void itm_init(void)` will only be called once during startup and doesn't have to be protected for multiple calls.
+- Targets must implement the function `void itm_init(void)` and add `ITM` to the `device_has` section in `target.json`.
+- When `void itm_init(void)` is called, the debug clock for the ITM must be initialized and the SWO pin configured for debug output.
+- `void itm_init(void)` only has to modify the clock prescaler in the generic register `TPI->ACPR`. The helper function `mbed_itm_init` initializes the generic ITM registers. 
+- `void itm_init(void)` is only called once during startup and doesn't have to be protected for multiple calls.
 
 ##### Undefined behavior
 
-* The debug clock frequency is left undefined since the most optimal frequency will vary from target to target. It is up to each target's owner to pick a frequency that (a) doesn't interfere with normal operation and (b) is supported by the owner's preferred debug monitor. 
+- The debug clock frequency is left undefined because the most optimal frequency varies from target to target. It is up to each target's owner to choose a frequency that doesn't interfere with normal operation and that the owner's preferred debug monitor supports. 
 
 #### Testing
 
-The `SerialWireOutput` class can be used for sending `stdout` to the SWO stimulus port on the ITM by including this override function:
+You can use the `SerialWireOutput` to send `stdout` to the SWO stimulus port on the ITM by including this override function:
 
 ```
 #include "SerialWireOutput.h"
@@ -31,4 +31,4 @@ FileHandle* mbed::mbed_override_console(int fd) {
 }
 ```
 
-The function can be placed in any C++ compilation unit, including `main.cpp`.
+You can place the function in any C++ compilation unit, including `main.cpp`.