docs: rectify title of no-unnecessary-waiting.md

[jquinc30]

Title of no-unnecessary-waiting.md was wrong so I update it from

No Assigning Return Values

to

No Unnecessary Waiting

[DamienCassou]

I've forked the project and welcome PRs: #100.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[nagash77]

Closing as this seems to already be updated.