Fix condition in _VimuxTmuxIndex()

This ensure that we call `_VimuxRunnerType()` instead of
`_VimuxRunnerType`.

Author: hovsater

plugin/vimux.vim

@@ -124,7 +124,7 @@ function! _VimuxTmuxSession()
 endfunction
 
 function! _VimuxTmuxIndex()
-  if _VimuxRunnerType == "pane"
+  if _VimuxRunnerType() == "pane"
     return _VimuxTmuxPaneIndex()
   else
     return _VimuxTmuxWindowIndex()