github: update pull request template

Update the pull request template to suggest providing a link to the
SV/LIBHTP pull request as this aides in cross linking.

Also change the way to provide the link to the Redmine ticket, as it
often results in linking to our Redmine, and add a checkbox for the
creation of a ticket.

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,8 +5,11 @@ Make sure these boxes are signed before submitting your Pull Request -- thank yo
 - [ ] I have signed the Open Information Security Foundation contribution agreement at
    https://suricata.io/about/contribution-agreement/ (note: this is only required once)
 - [ ] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)
+- [ ] I have created a ticket at
+      https://redmine.openinfosecfoundation.org/projects/suricata/issues
+      (if applicable)
 
-Link to [redmine](https://redmine.openinfosecfoundation.org/projects/suricata/issues) ticket:
+Link to ticket: https://redmine.openinfosecfoundation.org/issues/
 
 Describe changes:
 -
@@ -15,17 +18,13 @@ Describe changes:
 
 ### Provide values to any of the below to override the defaults.
 
-To use a pull request use a branch name like `pr/N` where `N` is the
-pull request number.
+- To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
+  link to the pull request in the respective `_BRANCH` variable.
+- Leave unused overrides blank or remove.
 
-Alternatively, `SV_BRANCH` may also be a link to an
-OISF/suricata-verify pull-request.
-
-```
 SV_REPO=
 SV_BRANCH=
 SU_REPO=
 SU_BRANCH=
 LIBHTP_REPO=
 LIBHTP_BRANCH=
-```