Update developing.adoc

[sbernabe1]

Updating git clone command to clone appropriate Repo

Adding Reviewer:
@synqotik

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[synqotik]

I'm thinking the developer should just clone the whole repo, then switch to the branch they need to work on, see my comment.

Eventually developers may work off of v9 or v12 branches.

[synqotik]

We may want to change this to something like:

. Use Git to check out the repository:
+
[source, shell]
git clone https://github.com/OpenNMS/grafana-plugin.git opennms-grafana-plugin

. Navigate to your plugin directory:
+
[source, shell]
cd opennnms-grafana-plugin

. Switch to the branch you want to work on. For OPG compatible with Grafana 9-11, you should work in the `release-9.x` branch:
+
[source, shell]
git branch release-9.x
git pull

. If you plan on making changes or making a PR, you should make a branch off of the main release branch:
+
[source, shell]
git checkout -b myusername/my-feature-name