chore: references to "googleapis" master branch

[nicain]

During the master --> main migration, I noticed references that point to the master branch of https://github.com/googleapis/googleapis

After that repo is migrated, we will need to circle back to this repo (and potentially others) and fix these references

• google/api_core/exceptions.py
• google/api_core/operations_v1/operations_async_client.py
• google/api_core/operations_v1/operations_client.py

xref: googleapis/google-cloud-python#10579

[vchudnov-g]

Of the three files listed, it seems only the first one refers to main in comment strings. Should be an easy fix...once we migrate master to main in googleapis/googleapis, which we still haven't done.

Alternatives:

• just link to the SHA rather than to the branch name. This is probably more robust, particularly for those references that link with a line number. That is very likely to change if we don't link to a particular commit
  OR
• remove the links altogether if they don't add value

[vchudnov-g]

Update: We will assess completing this task in relation to our other priorities.