qa: fix .teuthology_branch file in qa/

qa: fix .teuthology_branch file in qa/

According to teuthology-suite:

  -t <branch>, --teuthology-branch <branch>
                              The teuthology branch to run against.
                              Default value is determined in the next order.
                              There is TEUTH_BRANCH environment variable set.
                              There is `qa/.teuthology_branch` present in
                              the suite repo and contains non-empty string.
                              There is `teuthology_branch` present in one of
                              the user or system `teuthology.yaml` configuration
                              files respectively, otherwise use `main`.

The .teuthology_branch file in the qa/ dir currently points at "master".
Change it to point to "main".

Signed-off-by: Jeff Layton <jlayton@redhat.com>