fix(ft): don't use double semicolon for fennel (#304)

This PR replaces the double semicolons i.e., `;;%s` with
single semicolon i.e., `;%s` for [fennel](https://fennel-lang.org) files.

Author: atchim

lua/Comment/ft.lua

@@ -60,7 +60,7 @@ local L = setmetatable({
     elm = { M.dash, M.haskell_b },
     elvish = { M.hash },
 	faust = { M.cxx_l, M.cxx_b },
-    fennel = { M.lisp_l },
+    fennel = { ';%s' },
     fish = { M.hash },
     fsharp = { M.cxx_l, M.fsharp_b },
     gdb = { M.hash },