Fix bug when updating with xdotool in local folder

As html with images is rendered into the local folder, the local preview
function is used.

However a ' was missing in the call of `xdotool search`, spawning new
windows with every call (independent of the browser option).

Closes JamshedVesuna#84.

plugin/vim-markdown-preview.vim

@@ -129,7 +129,7 @@ function! Vim_Markdown_Preview_Local()
   endif
 
   if g:vmp_osname == 'unix'
-    let chrome_wid = system("xdotool search --name vim-markdown-preview.html - " . g:vim_markdown_preview_browser . "'")
+    let chrome_wid = system("xdotool search --name 'vim-markdown-preview.html - " . g:vim_markdown_preview_browser . "'")
     if !chrome_wid
       if g:vim_markdown_preview_use_xdg_open == 1
         call system('xdg-open vim-markdown-preview.html 1>/dev/null 2>/dev/null &')