doc: improve child_process.markdown wording
doc: improve child_process.markdown wording

PR-URL: https://github.com/nodejs/node/pull/4138
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Minwoo Jung <8ed4d68b4c6f320972a8484fc5a67b9b0188ed87@gmail.com>
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node