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: #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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,9 +13,7 @@ Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 - [ ] `make -j4 test` (UNIX), or `vcbuild test nosign` (Windows) passes
 - [ ] tests and/or benchmarks are included
 - [ ] documentation is changed or added
-- [ ] commit message follows [commit guidelines][]
+- [ ] commit message follows [commit guidelines](https://github.com/nodejs/node/blob/master/CONTRIBUTING.md#commit-guidelines)
 
 ##### Affected core subsystem(s)
 <!-- Provide affected core subsystem(s) (like doc, cluster, crypto, etc). -->
-
-[commit guidelines]: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md#commit-guidelines