database/sql: refer correct issue in TestErrBadConnReconnect comment

Change-Id: I283ab238b60d3a47e86296e1fbfc73ba121bef19
Reviewed-on: https://go-review.googlesource.com/10745Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -1458,7 +1458,7 @@ func TestManyErrBadConn(t *testing.T) {
 	}
 }
 
-// golang.org/issue/5781
+// golang.org/issue/5718
 func TestErrBadConnReconnect(t *testing.T) {
 	db := newTestDB(t, "foo")
 	defer closeDB(t, db)