Copy edit storage.md

Make minor copy edits for active voice, consistent spelling and proper punctuation.

Author: Amanda Butler

docs/reference/contributing/storage/storage.md

@@ -1,16 +1,16 @@
 <h2 id="contributing-storage">Storage</h2>
 
-Storage support is split between filesystems and their underlying block device support. The [storage API page](/docs/v5.6/reference/storage-overview.html) has more information on existing APIs in Mbed OS for both interfaces.
+Storage support is split between file systems and their underlying block device support. The [storage API page](/docs/v5.6/reference/storage-overview.html) has more information on existing APIs in Mbed OS for both interfaces.
 
 #### Block Device
 
-Adding a block device implementation is required for backing filesystems on new hardware. You can extend the [BlockDevice](https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/bd/HeapBlockDevice.h) class to provide support for unsupported storage. 
+Backing file systems on new hardware requires adding a block device implementation. You can extend the [BlockDevice](https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/bd/HeapBlockDevice.h) class to provide support for unsupported storage.
 
-If you want to port a new filesystem to Mbed OS on existing storage options you can skip to the following section.
+If you want to port a new file system to Mbed OS on existing storage options, you can skip to the following section.
 
-#### Filesystems
+#### File systems
 
-To implement a new file system in Mbed OS, an implementor needs to provide the abstract functions in the file system interface. The [FAT file system](https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/fat/FATFileSystem.cpp) provides an excellent example, you can find tests of the POSIX API here.
+To implement a new file system in Mbed OS, an implementor needs to provide the abstract functions in the file system interface. The [FAT file system](https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/fat/FATFileSystem.cpp) provides an excellent example. You can find tests of the POSIX API here.
 
 A minimal file system needs to provide the following functions:
 
@@ -20,8 +20,8 @@ A minimal file system needs to provide the following functions:
 - `file_write`.
 - `file_seek`.
 
-Here is the full API that a filesystem may implement:
+Here is the full API that a file system may implement:
 
 [![View code](https://www.mbed.com/embed/?type=library)](https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/FileSystem.h#L205)
 
-Filesystems must be backed by a block device in Mbed OS. If you are using supported hardware then you can continue, otherwise view the block device porting section above.
+A block device must back file systems in Mbed OS. If you are using supported hardware, then you can continue. Otherwise, view the block device porting section above.