add fp< to undo_ftplugin

Author: faerryn

ftplugin/zig.vim

@@ -46,7 +46,7 @@ endif
 let &l:formatprg = 'zig fmt --stdin'
 
 let b:undo_ftplugin =
-    \ 'setl isk< et< ts< sts< sw< fo< sua< mp< com< cms< inex< inc< pa<'
+    \ 'setl isk< et< ts< sts< sw< fo< sua< mp< com< cms< inex< inc< pa< fp<'
 
 let &cpo = s:cpo_orig
 unlet s:cpo_orig