Fixes two typos in this measurements.rst

Adds "s" for two meters "disk.read.request" and "disk.write.request"
modified:   doc/source/measurements.rst

Fixes bug #1220176

Change-Id: I45295e76ac49e4bad1253c5dca9d8ed65e5764e3
(cherry picked from commit 472312a1622ea0e63324b251f5117f47ca6fee1f)
Merge "Fixes two typos in this measurements.rst" into stable/grizzly