[llvm] [Github] Tweak the good-first-issue message not to require assignment. (PR #139858)

[Erich Keane via llvm-commits]

https://github.com/erichkeane commented:

I really like this!  I've found that this is quite successful in the FE.  The assignment chases folks away, so someone that says "assign me" then disappears ends up discouraging contribution.

So I'm in favor, and hope that we're willing to accept this.

https://github.com/llvm/llvm-project/pull/139858