percpu-rw-semaphore.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

This document is already adopting the standard format,
with a single exception: we're using this convention
for the document title:
===
foo
===

So, adjust the title of this document to follow the
standard.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/percpu-rw-semaphore.txt

···
+====================
 Percpu rw semaphores
---------------------
+====================
 
 Percpu rw semaphores is a new read-write semaphore design that is
 optimized for locking for reading.