refactor/rename: add missing operand to fmt.Errorf

TBR=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/177920043

refactor/rename/spec.go

@@ -197,7 +197,7 @@ func parseObjectSpec(spec *spec, main string) error {
 		}
 	}
 
-	return fmt.Errorf("-from %q: invalid expression")
+	return fmt.Errorf("-from %q: invalid expression", main)
 }
 
 // parseImportPath returns the import path of the package denoted by e.