Typo fix

Author: drewcassidy

docs/tools/mbed_targets.md

@@ -199,7 +199,7 @@ The `default_toolchain` property names the toolchain that compiles code for this
 
 #### `post_binary_hook`
 
-Some targets require specific actions to generate a programmable binary image. Specify these actions using the `post_binary_hook` property and custom Python code. The value of `post_binary_hook` must be a JSON object with keys `function` and optionally `toolchain`. Within the `post_binary_hook` JSON object, the `function` key must contain a Python function that is accessible from the namespace of `tools/targets/__init__.py`, and the optional `toolchain` key must contain a list of toolchains that require processing from the `post_binary_hook`. When you do not specify the `toolchains` key for a `post_binary_hook`, you can assume the `post_binary_hook` applies to all toolcahins. For the `TEENSY3_1` target above, the definition of `post_binary_hook` looks like this:
+Some targets require specific actions to generate a programmable binary image. Specify these actions using the `post_binary_hook` property and custom Python code. The value of `post_binary_hook` must be a JSON object with keys `function` and optionally `toolchain`. Within the `post_binary_hook` JSON object, the `function` key must contain a Python function that is accessible from the namespace of `tools/targets/__init__.py`, and the optional `toolchain` key must contain a list of toolchains that require processing from the `post_binary_hook`. When you do not specify the `toolchains` key for a `post_binary_hook`, you can assume the `post_binary_hook` applies to all toolchains. For the `TEENSY3_1` target above, the definition of `post_binary_hook` looks like this:
 
 ```json
 "post_binary_hook": {