[Term Entry] C++ Unordered-sets: key_eq()

[codecademy-docs]

Reason/inspiration (optional)

We would like a new entry on the term key_eq() under unordered-sets in C++. The entry should be in content/cpp/concepts/unordered-set/terms/key-eq/key-eq.md

The entry should include the following:

• A description of the term
• A ##Syntax section that includes the syntax of the method and its details
• An ##Example section that show an example of the current entry.
• An ## Codebyte section that has a compilable code inside it showing the current entry in use. Use ```codebyte/cpp to add compilable code.

Please refer to the term entry template, content standards and markdown style guide when working on the PR for this issue.

Entry Type (select all that apply)

New Term for Existing Concept Entry

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