centos6.rst: fix reStructuredText syntax

[maxim-belkin]

fix reStructuredText syntax:

• use .. code-block:: bash instead of literal blocks.
• replace " with `` where appropriate
• Use .. note:: for notes

[scanon]

The test are failing for a different reason. Since this is a docs only update, I'm going to merge it.

[coveralls]

Coverage increased (+0.09%) to 78.352% when pulling 3a345d6b427bf7ebf3335c7ed623d492032285b3 on maxim-belkin:centos6-rst into 169d1a298a02418e80c590b63ff5bfa6626c5ac1 on NERSC:master.