Personalize MISSING EXPRESSION message

[landongrindheim]

Quick Summary:
I encountered this error message when playing around in elm repl. I had to re-read the sentence a couple times to make sure I understood what the compiler was telling me. I hope this small change makes the message just a little easier to understand 😄

I also updated the link to the ContributorAgreement as I encountered a 404 when I decided to submit this change. Let me know if you'd like that to be in a separate pull request 🙂

[landongrindheim]

I'm not sure why this is failing the Set Pull Expectation action. I tried this earlier (#2137) and had the same result. If you have insight, please let me know.

Update: I think this is because I'm opening a PR from a fork and don't have write access to this repository.

[evancz]

Ah, thank you! Much better with that word filled in!

I have learned that the mere act of using GitHub to do contributions has the contributed code fall under the stated license of the project. I think that wasn't as clear legally in the 90s (which is the era of the open source person I initially got my recommendations from) so I need to update files here and there to indicate that we are using the inbound=outbound method for this stuff.