Remove unnecessary backticks; ref #4301

mrbgems/mruby-method/mrblib/kernel.rb

@@ -3,7 +3,7 @@ module Kernel
     m = method(name)
     sc = (class <<self; self; end)
     if m.owner != sc
-      raise NameError, "undefined method `#{name}' for class `#{sc}'"
+      raise NameError, "undefined method '#{name}' for class '#{sc}'"
     end
     m
   end

mrbgems/mruby-method/src/method.c

@@ -327,7 +327,7 @@ name_error:
   s = mrb_class_name(mrb, c);
   mrb_raisef(
     mrb, E_NAME_ERROR,
-    "undefined method `%S' for class `%S'",
+    "undefined method '%S' for class '%S'",
     mrb_sym2str(mrb, name),
     mrb_str_new_static(mrb, s, strlen(s))
   );