PR template: remove outdated contributing guide link

update contributing guide link and improve PR template formatting in `.github/PULL_REQUEST_TEMPLATE.md`

Signed-off-by: Paulchen <lukas.23022005@gmail.com>
Signed-off-by: Lukas Michael <lukas.23022005@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: paulchen5

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,9 +4,6 @@ https://github.com/docker/cli/blob/master/CONTRIBUTING.md
 
 ** Make sure all your commits include a signature generated with `git commit -s` **
 
-For additional information on our contributing process, read our contributing
-guide https://docs.docker.com/opensource/code/
-
 If this is a bug fix, make sure your description includes "fixes #xxxx", or
 "closes #xxxx"
 
@@ -20,6 +17,7 @@ Provide the following information:
 **- How to verify it**
 
 **- Human readable description for the release notes**
+
 <!--
 Write a short (one line) summary that describes the changes in this
 pull request for inclusion in the changelog.
@@ -28,10 +26,9 @@ It must be placed inside the below triple backticks section.
 NOTE: Only fill this section if changes introduced in this PR are user-facing.
 The PR must have a relevant impact/ label.
 -->
-```markdown changelog
 
+```markdown changelog
 
 ```
 
 **- A picture of a cute animal (not mandatory but encouraged)**
-