Fix help formatting for string and filename options

I happened to notice that "help add-inferior" said:

  -execFILENAME
    FILENAME is the file name of the executable to use as the
    main program.

This is missing a space after "-exec".  This patch fixes the bug.

If ok'd on time I plan to check this in to the gdb-16 branch as well.

Approved-by: Kevin Buettner <kevinb@redhat.com>