docs Update runtime.md

"period of time" is redundant. It would be better to say only period, because all periods are periods of time.

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Issue Solved

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Adding a new entry
Editing an existing entry (fixing a typo, bug, issues, etc)
Updating the documentation

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[ ] All writings are my own.
[ ] My entry follows the Codecademy Docs style guide.
[ ] My changes generate no new warnings.
[ ] I have performed a self-review of my own writing and code.
[ ] I have checked my entry and corrected any misspellings.
[ ] I have made corresponding changes to the documentation if needed.
[ ] I have confirmed my changes are not being pushed from my forked main branch.
[ ] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
[ ] I have linked any issues that are relevant to this PR in the Issues Solved section.

Apr 15 '24 15:04 ARBUCHELI

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