RFR: 2401: Link to Webrev should be part of the Reviewing section

[Magnus Ihse Bursie]

When we added the link to the webrev comment in the PR body, for some reason it got its own header `### Webrev`. But Webrevs are just another way of reviewing, so it should be put under the `### Reviewing` header.

That is, the reviewing html code should look something like this:

...
<details><summary>Using diff file</summary>

Download this PR as a diff file: \
<a href="https://git.openjdk.org/jdk/pull/XXX.diff">[https://git.openjdk.org/jdk/pull/XXX.diff</a](https://git.openjdk.org/jdk/pull/XXX.diff%3C/a)>

</details>
<details><summary>Using Webrev</summary>

[Link to Webrev Comment](https://git.openjdk.org/jdk/pull/XXX#issuecomment-YYY)

</details>

-------------

Commit messages:
 - 2401: Link to Webrev should be part of the Reviewing section

Changes: https://git.openjdk.org/skara/pull/1695/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1695&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2401
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1695.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1695/head:pull/1695

PR: https://git.openjdk.org/skara/pull/1695