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>
diff --git a/Documentation/filesystems/sharedsubtree.txt b/Documentation/filesystems/sharedsubtree.txt
index 32a173d..e3f4c77 100644
--- a/Documentation/filesystems/sharedsubtree.txt
+++ b/Documentation/filesystems/sharedsubtree.txt
@@ -664,7 +664,7 @@
 		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