Minor fix to the FAQ

This is awkward to describe, sorry. :)

The old setting doesn't work when Vim reads a file from stdin, f.i.:

    diff file1 file2 | vim -

In this situation `argc() == 0`, since no file has been named on the
command line, yet we don't want to open NERDTree, as we already
have a file to play with.  However, when Vim reads a file from stdin
the buffer is still marked as modified.  Hence the patch.