doc: consistent 'Returns:' part two

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

Follow up from 8eb19c48. Lower case return was not updated

Ref: https://github.com/nodejs/node/pull/9554