Default GITHUB_BRANCH for sbs-config.json is incorrect

The default that has been set for GITHUB_BRANCH (in the documentation - https://github.com/ibm-hyper-protect/secure-build-cli/blob/master/SBS-HPVScloud.md#preparing-the-configuration) is set to **master**

Github no longer uses master as the default branch - this has shifted to **main**

Just for ease of use purposes I think it would be worthwhile changing this.



