Documentation: fix formatting to make 's' happy

"That letter [the last s] is sad because all the others
have those things [=] below them and it does not."

This patch fixes the tragedy so all the letters can
be happy again.
Signed-off-by: NMaisa Roponen <maisa.roponen@gmail.com>
[The author being 4 years old needed some assistance]
Signed-off-by: NTero Roponen <tero.roponen@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/proc.txt

@@ -1272,7 +1272,7 @@ softirq.
 
 
 1.9 Ext4 file system parameters
-------------------------------
+-------------------------------
 
 Information about mounted ext4 file systems can be found in
 /proc/fs/ext4.  Each mounted filesystem will have a directory in