Make cephx key creation syntax consistent

These are a couple of simple fixes to clarify things when creating or updating cephx keys.

I found one place where the example was not consistent with every other example on the page by not using "osd 'allow rw'" syntax, isntead it was just "osd rw".  That was confusing.

The second bit clarifies that a pool=<pool> specification should be inside of single quotes.

I ran across this when trying to create a new key and limit permissions to a single pool.  The docs here on CephX were not as useful about how to do that as the Block Devices/OpenStack page is.  This should bring them in parity, I think.