Fix accidentally using a symbol as a format string.

If the symbol happens to contain a percent (%), the following happend:

    File "$HOME/.config/sublime-text-3/Packages/CTags/ctagsplugin.py", line 412, in format_tag_for_quickopen
      format = [(f or tag.symbol) % tag, tag.ex_command]
  ValueError: incomplete format
Merge pull request #264 from nh2/development-fix-format-string-exception

Fix accidentally using a symbol as a format string.