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

Documentation/vm/slub.txt

@@ -8,7 +8,7 @@ SLUB can enable debugging only for selected slabs in order to avoid
 an impact on overall system performance which may make a bug more
 difficult to find.
 
-In order to switch debugging on one can add a option "slub_debug"
+In order to switch debugging on one can add an option "slub_debug"
 to the kernel command line. That will enable full debugging for
 all slabs.