Use right syntax for symbolic-ref command (#21577)

Follow-up to #21352: when testing this out, I found out that the syntax
for the `symbolic-ref` command was different than originally assumed in
that PR. Never got around to submitting a PR until now.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/doctor/heads.go

@@ -49,7 +49,7 @@ func synchronizeRepoHeads(ctx context.Context, logger log.Logger, autofix bool)
 		}
 
 		// otherwise, let's try fixing HEAD
-		err := git.NewCommand(ctx, "symbolic-ref").AddDashesAndList("HEAD", repo.DefaultBranch).Run(runOpts)
+		err := git.NewCommand(ctx, "symbolic-ref").AddDashesAndList("HEAD", git.BranchPrefix+repo.DefaultBranch).Run(runOpts)
 		if err != nil {
 			logger.Warn("Failed to fix HEAD for %s/%s: %v", repo.OwnerName, repo.Name, err)
 			return nil