Adding OptIn to the allow list for spell check

.github/workflows/codespell.yml

@@ -13,4 +13,4 @@ jobs:
         with:
           check_filenames: true
           skip: ./.git,./conformance/third_party,*.snk,*.pb,*.pb.cc,*.pb.h,./src/google/protobuf/testdata,./objectivec/Tests,./python/compatibility_tests/v2.5.0/tests/google/protobuf/internal
-          ignore_words_list: "alow,alse,ba,cleare,copyable,cloneable,dedup,dur,errorprone,files',fo,fundementals,hel,importd,inout,leapyear,nd,nin,ois,ons,parseable,process',te,testof,ue,unparseable,wasn,wee,gae,keyserver,objext,od"
+          ignore_words_list: "alow,alse,ba,cleare,copyable,cloneable,dedup,dur,errorprone,files',fo,fundementals,hel,importd,inout,leapyear,nd,nin,ois,ons,parseable,process',te,testof,ue,unparseable,wasn,wee,gae,keyserver,objext,od, OptIn"