doc: update pull request template URL layout

Putting the link for the commit guideline at the bottom of the pull
request template makes it a bit confusing when opening a pull request as
the markdown does not render. Not everyone is familiar with the syntax
used there and it is easy to mess it up (for example, by not leaving a
blank line above it). It is more natural and useful if the URL is
included inline.

PR-URL: https://github.com/nodejs/node/pull/12216


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>