Improve common string mapping instruction

doc/cmp.txt

@@ -211,9 +211,7 @@ NOTE: You can call these functions in mapping via `<Cmd>lua require('cmp').compl
     mapping = {
       ['<C-l>'] = cmp.mapping(function(fallback)
         if cmp.visible() then
-          if cmp.complete_common_string() then
-            return
-          end
+          return cmp.complete_common_string()
         end
         fallback()
       end, { 'i', 'c' }),

lua/cmp/core.lua

@@ -249,7 +249,7 @@ core.complete_common_string = function(self)
       common_string = str.get_common_string(common_string, vim_item.word)
     end
   end
-  if common_string and #common_string >= (1 + cursor[2] - offset) then
+  if common_string and #common_string > (1 + cursor[2] - offset) then
     feedkeys.call(keymap.backspace(string.sub(api.get_current_line(), offset, cursor[2])) .. common_string, 'n')
     return true
   end