Update generator to reference links on main branch of gax rather than master

[bcoe]

Once google-gax is migrated to using the main branch, references to documentation in the gax repository, created during code generation, will need to be updated to reference the main branch.

Refs: #360

[alexander-fenster]

https://github.com/googleapis/gapic-generator-typescript/blob/main/templates/typescript_gapic/_util.njk#L36

[alkatrivedi]

Hi Team, we are encountering a failure in the docs-test for the Nodejs Spanner library. The failure is because of following broken link:
https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md

I made a temporary fix in this PR, where the link was manually updated to https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md. However, as the affected files are autogenerated, this manual change will be overwritten by OwlBot during the next repository synchronization, causing the docs-test to fail again.

Therefore, I am requesting a lasting fix, which can be addressed by implementing this feature. Could you please examine this ?

cc: @alexander-fenster @sofisl @surbhigarg92