Don't set help files to nomodifiable.

It's really neat to have R help files open up right in a vim buffer if for no other reason than the ability to very easily send the examples in the help files to the R console like any other R file.  However, since the help files are set to nomodifiable, you can't change the examples if you want to experiment unless you take some extra steps and copy into a modifiable buffer.  This PR sets the help files to readonly instead of nomodifiable.