doc: use _code name_ rather than _codename_
This is excrutiatingly minor, but every authoritative resource I've checked (which is to say, a few dictionaries) uses "code name" primarily or exclusively. Let's follow suit.
Related Issues
Fixes: https://github.com/nodejs/node/issues/<issue_number>
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines