correct mutating functions

[depial]

I've added some more code examples in instructions.md to make clear that title is a mutating function.

Also, I have changed the names of the functions title and revive to title! and revive! (to reflect that they are indeed mutating functions) in role-playing-game.jl and runtests.jl. I've made runtests.jl backwards compatible, so the current solutions won't break since I think it would be pointless to make everyone copy and paste their current solutions with new function names. The change in exemplar.jl will come in following commit.

This PR should take [no important files changed] in the merge commit body.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[depial]

Working on backwards compatibility...

[colinleach]

Thanks for tackling this. Good luck!

[depial]

I needed to leave the new assignment in the global scope, so this should be backwards compatible now.

[depial]

Great! Looks like it's working on the site. Both of my solutions, with the old and new function names, work.