style(doctor): use s.Error for "No fork point stored" message

Consistent with other simple issue messages in checkBranch that
use the style package for formatting.

Author: cds-amal

cmd/doctor.go

@@ -147,7 +147,7 @@ func checkBranch(g *git.Git, cfg *config.Config, s *style.Style, branch string,
 			result.fixMsg = fmt.Sprintf("set fork point to %s", git.AbbrevSHA(newFP))
 			return result
 		}
-		result.issues = append(result.issues, "No fork point stored")
+		result.issues = append(result.issues, s.Error("No fork point stored"))
 		return result
 	}