Corrected supported_c_libs

Author: evedon

docs/reference/bare_metal/microlib.md

@@ -22,10 +22,10 @@ By default, only a few targets have been tested with microlib. If your target ha
 
 Once you have completed the steps above, add `small` to the `supported_c_libs` parameter for your target in `targets.json`:
 ```
-"supported_c_lib": {
-    "ARM": ["std", "small"],
-    "GCC": ["std", "small"],
-    "IAR": ["std"]
+"supported_c_libs": {
+    "arm": ["std", "small"],
+    "gcc_arm": ["std", "small"],
+    "iar": ["std"]
 }
 ```
 
@@ -35,7 +35,7 @@ You can build with microlib by creating a `mbed_app.json` with the following con
  ```
  {
   "target_overrides": {
-    "*": {  
+    "*": {
       "target.c_lib": "small"
     }
   }