docs: update contributing guide to rebase of origin/master not local master branch

Updating the squash command to reference origin/master instead of the local master branch in the event a local environment's master branch is not up to date.

chart/templates/gitlab/gitrepository.yaml

@@ -4,6 +4,10 @@ kind: GitRepository
 metadata:
   name: gitlab
   namespace: {{ .Release.Namespace }}
+  labels:
+    app.kubernetes.io/name: gitlab
+    app.kubernetes.io/component: "developer-tools"
+    {{- include "commonLabels" . | nindent 4}}
 spec:
   interval: {{ .Values.flux.interval }}
   url: {{ .Values.addons.gitlab.git.repo }}