new2 => new_cstr

tiny fix for this commit
↓
Merge pull request #984 from monaka/pr-make-mrb_str_new2-obsolete · e53628e · 
mruby/mruby
https://github.com/mruby/mruby/commit/e53628eb9d6ee5e5979dca9e686236ab58e3fd23