doc: fixes default shell in child_process.md
This PR clarifies that 'process.env.ComSpec'
is the default shell in
Windows and that 'cmd.exe'
is only used as a fallback.
Fixes: https://github.com/nodejs/node/issues/14156
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc