Update default branch name for Redfish Validator

Changes:
    - Update default master to main

Tested: - Tested from sandbox
Change-Id: Iac37626a9bc23aa1766c932745b68768e60b0fa5
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/redfish/dmtf_tools/Redfish_Service_Validator.robot b/redfish/dmtf_tools/Redfish_Service_Validator.robot
index ca54e41..9f0f499 100644
--- a/redfish/dmtf_tools/Redfish_Service_Validator.robot
+++ b/redfish/dmtf_tools/Redfish_Service_Validator.robot
@@ -16,7 +16,7 @@
 ${cmd_str_master}  ${DEFAULT_PYTHON} ${rsv_dir_path}${/}RedfishServiceValidator.py
 ...                --ip https://${OPENBMC_HOST}:${HTTPS_PORT} --authtype=Session -u ${OPENBMC_USERNAME}
 ...                -p ${OPENBMC_PASSWORD} --logdir ${EXECDIR}${/}logs${/} --debugging
-${branch_name}    master
+${branch_name}    main
 
 *** Test Case ***