Doc: treewide: Fix grammar "a" to "an"

This patch fix some grammar mistake.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/sharedsubtree.txt

@@ -664,7 +664,7 @@ replicas continue to be exactly same.
 		if one rbind mounts a tree within the same subtree 'n' times
 		the number of mounts created is an exponential function of 'n'.
 		Having unbindable mount can help prune the unneeded bind
-		mounts. Here is a example.
+		mounts. Here is an example.
 
 		step 1:
 		   let's say the root tree has just two directories with