Copy edit FATFileSystem.md

Copy edit file for consistent person across documents and minor grammar nits.

Author: Amanda Butler

docs/reference/api/storage/FATFileSystem.md

@@ -20,7 +20,7 @@ The first step to using the FAT file system is formatting storage with FAT. You
 
 The FAT file system supports external flash; however, it must allocate a full erase block for internal operations, which can become large for some forms of flash. If RAM consumption becomes a problem, we suggest switching to LittleFileSystem. The Mbed OS file system APIs make switching file systems a straightforward task. One common strategy is to use the FAT file system for debugging and switch to LittleFileSystem when the application becomes stable.
 
-The FAT file system is thread safe. Although FAT file system is built on the ChanFS project, synchronizing in the C++ layer allows us to directly use the C++ RTOS APIs.
+The FAT file system is thread safe. Although the FAT file system is built on the ChanFS project, synchronizing in the C++ layer allows you to use the C++ RTOS APIs directly.
 
 ### Usage