Documentation/filesystems: Fixed typo

Fixed a -> an typo.

Signed-off-by: Robert Foss <robert.foss@collabora.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/proc.txt

@@ -145,7 +145,7 @@ Table 1-1: Process specific entries in /proc
 		symbol the task is blocked in - or "0" if not blocked.
  pagemap	Page table
  stack		Report full stack trace, enable via CONFIG_STACKTRACE
- smaps		a extension based on maps, showing the memory consumption of
+ smaps		an extension based on maps, showing the memory consumption of
 		each mapping and flags associated with it
  numa_maps	an extension based on maps, showing the memory locality and
 		binding policy as well as mem usage (in pages) of each mapping.