doc: fix inconsistency in documentation for building
Specifically, fix the inconsistency where the documentation suggests running "$ make test" instead of "$ make -j4 test". The "-j4" flag uses multiple processes, making the command faster.
Fixes: https://github.com/nodejs/node/issues/19919
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines