Change branch from matser to main for postsubmit-os-origin-build-golang-master-ppc64le job

Author: Rajalakshmi-Girish

config/jobs/ppc64le-cloud/builds/openshift-postsubmit.yaml

@@ -64,7 +64,7 @@ postsubmits:
         - master
       run_if_changed: '^golang/master/'
       extra_refs:
-        - base_ref: master
+        - base_ref: main
           org: openshift
           repo: origin
           workdir: true