Rename `mrb_str_concat2` to `mrb_str_concat_m`.

According to the naming convention a function that implements
a method should be suffixed by `_m`.

mrbgems/mruby-string-ext/src/string.c

@@ -152,7 +152,7 @@ static mrb_value mrb_fixnum_chr(mrb_state *mrb, mrb_value num);
  *     a.concat(33)   #=> "hello world!"
  */
 static mrb_value
-mrb_str_concat2(mrb_state *mrb, mrb_value self)
+mrb_str_concat_m(mrb_state *mrb, mrb_value self)
 {
   mrb_value str;
 
@@ -661,8 +661,8 @@ mrb_mruby_string_ext_gem_init(mrb_state* mrb)
   mrb_define_method(mrb, s, "byteslice",       mrb_str_byteslice,       MRB_ARGS_REQ(1)|MRB_ARGS_OPT(1));
   mrb_define_method(mrb, s, "swapcase!",       mrb_str_swapcase_bang,   MRB_ARGS_NONE());
   mrb_define_method(mrb, s, "swapcase",        mrb_str_swapcase,        MRB_ARGS_NONE());
-  mrb_define_method(mrb, s, "concat",          mrb_str_concat2,         MRB_ARGS_REQ(1));
-  mrb_define_method(mrb, s, "<<",              mrb_str_concat2,         MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, s, "concat",          mrb_str_concat_m,        MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, s, "<<",              mrb_str_concat_m,        MRB_ARGS_REQ(1));
   mrb_define_method(mrb, s, "start_with?",     mrb_str_start_with,      MRB_ARGS_REST());
   mrb_define_method(mrb, s, "end_with?",       mrb_str_end_with,        MRB_ARGS_REST());
   mrb_define_method(mrb, s, "hex",             mrb_str_hex,             MRB_ARGS_NONE());