Fix Javadoc for DatabaseClient

The documentation for DatabaseClient had misplaced double quotes in the
example on how to map a select statement to a row.

Closes gh-28520

spring-r2dbc/src/main/java/org/springframework/r2dbc/core/DatabaseClient.java

@@ -45,8 +45,8 @@
  *
  * DatabaseClient client = DatabaseClient.create(factory);
  * Mono<Actor> actor = client.sql("select first_name, last_name from t_actor")
- *     .map(row -> new Actor(row.get("first_name, String.class"),
- *          row.get("last_name, String.class")))
+ *     .map(row -> new Actor(row.get("first_name", String.class),
+ *          row.get("last_name", String.class)))
  *     .first();
  * </pre>
  *