README: fix example for database/sql (#955)

README.md

@@ -342,7 +342,7 @@ import (
 func main() {
 	// dsn format: "user:password@addr?dbname"
 	dsn := "[email protected]:3306?test"
-	db, _ := sql.Open(dsn)
+	db, _ := sql.Open("mysql", dsn)
 	db.Close()
 }
 ```