Update documentation to reflect preference for Lettuce

See gh-886

spring-session-data-redis/src/main/java/org/springframework/session/data/redis/RedisOperationsSessionRepository.java

@@ -64,7 +64,7 @@ import org.springframework.util.Assert;
  * A typical example of how to create a new instance can be seen below:
  *
  * <pre>
- * JedisConnectionFactory factory = new JedisConnectionFactory();
+ * LettuceConnectionFactory factory = new LettuceConnectionFactory();
  *
  * RedisOperationsSessionRepository redisSessionRepository = new RedisOperationsSessionRepository(factory);
  * </pre>

spring-session-data-redis/src/main/java/org/springframework/session/data/redis/config/annotation/web/http/EnableRedisHttpSession.java

@@ -38,8 +38,8 @@ import org.springframework.session.data.redis.RedisFlushMode;
  * public class RedisHttpSessionConfig {
  *
  *     {@literal @Bean}
- *     public JedisConnectionFactory connectionFactory() throws Exception {
- *         return new JedisConnectionFactory();
+ *     public LettuceConnectionFactory connectionFactory() {
+ *         return new LettuceConnectionFactory();
  *     }
  *
  * }