Minor edits

Changed reference of file.open() to just open(), because it has been several
years since file.open() was a valid call.  Also, added a missing word.  It looks
like the document is up to date otherwise.
Merge pull request #478 from lydia-duncan/docFreezeIO

Minor edits to README.io [not reviewed]

Changed reference of file.open() to just open(), because it has been several
years since file.open() was a valid call. Also, added a missing word. It looks
like the document is up to date otherwise.