Minor edits to README.hdfs

Added a comma, changed the tense of one verb to match the tense of those listed
in the previous steps, and changed some demonstrative pronouns to more specific
descriptions for greater clarity.
Merge pull request #494 from lydia-duncan/docFreezeHDFS

Minor edits to README.hdfs [reviewed by Ben]

Added a comma, changed the tense of one verb to match the tense of those listed
in the previous steps, and changed some demonstrative pronouns to more specific
descriptions for greater clarity.