docs: update README with examples for no-tx and advisory lock

Author: padurean

README.md

@@ -87,11 +87,11 @@ func main() {
             Version: 2,
             UpDownNoTX: &gosmig.UpDownNoTXSQL{
                 Up: func(ctx context.Context, db *sql.DB) error {
-                    _, err := db.ExecContext(ctx, `ALTER TABLE users ADD COLUMN age INT`)
+                    _, err := db.ExecContext(ctx, `CREATE INDEX CONCURRENTLY idx_users_created_at ON users (created_at)`)
                     return err
                 },
                 Down: func(ctx context.Context, db *sql.DB) error {
-                    _, err := db.ExecContext(ctx, `ALTER TABLE users DROP COLUMN age`)
+                    _, err := db.ExecContext(ctx, `DROP INDEX CONCURRENTLY IF EXISTS idx_users_created_at`)
                     return err
                 },
             },
@@ -166,11 +166,11 @@ func main() {
             Version: 2,
             UpDownNoTX: &UpDownNoTXSQLX{
                 Up: func(ctx context.Context, db *sqlx.DB) error {
-                    _, err := db.ExecContext(ctx, `ALTER TABLE products ADD COLUMN price DECIMAL(10,2)`)
+                    _, err := db.ExecContext(ctx, `CREATE INDEX CONCURRENTLY idx_users_created_at ON users (created_at)`)
                     return err
                 },
                 Down: func(ctx context.Context, db *sqlx.DB) error {
-                    _, err := db.ExecContext(ctx, `ALTER TABLE products DROP COLUMN price`)
+                    _, err := db.ExecContext(ctx, `DROP INDEX CONCURRENTLY IF EXISTS idx_users_created_at`)
                     return err
                 },
             },