Remove incorrect comment in comments

Remove the incorrect comment "\\" when setting comments in
ftplugin/zig.vim.

"\\" is used for multiline string literals.

Closes ziglang#74

Author: perillo

ftplugin/zig.vim

@@ -24,7 +24,7 @@ setlocal formatoptions-=t formatoptions+=croql
 setlocal suffixesadd=.zig,.zir
 
 if has('comments')
-    setlocal comments=:///,://!,://,:\\\\
+    setlocal comments=:///,://!,://
     setlocal commentstring=//\ %s
 endif