style: use single-quotes to resolve linter error

Author: kkoomen

autoload/doge.vim

@@ -77,7 +77,7 @@ endfunction
 " arg: Either a count (0 by default) or a string (empty by default).
 function! doge#generate(arg) abort
   if !exists('b:doge_supported_doc_standards')
-    echoerr "[vim-doge] It seems like you forgot to set `filetype plugin on` in your .vimrc"
+    echoerr '[vim-doge] It seems like you forgot to set `filetype plugin on` in your .vimrc'
     return 1
   endif