[SPARK-37692][DOCS] Fix an example of mixed interval fields in the SQL migration guide
[SPARK-37692][DOCS] Fix an example of mixed interval fields in the SQL migration guide

### What changes were proposed in this pull request?
Change a wrong example in the SQL migration guide, and align it to the description of behavior change.
<img width="1022" alt="Screenshot 2021-12-21 at 12 00 53" src="https://user-images.githubusercontent.com/1580697/146901704-c635720b-a8ca-4ea0-bc79-0220b83f544f.png">

### Why are the changes needed?
Current example is incorrect, and can confuse users.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
By running the example:
```sql
spark-sql> select INTERVAL 1 day 1 hour;
1 01:00:00.000000000
spark-sql> select INTERVAL 1 month 1 hour;
Error in query:
Cannot mix year-month and day-time fields: INTERVAL 1 month 1 hour(line 1, pos 7)

== SQL ==
select INTERVAL 1 month 1 hour
-------^^^

spark-sql> set spark.sql.legacy.interval.enabled=true;
spark.sql.legacy.interval.enabled	true
spark-sql> select INTERVAL 1 month 1 hour;
1 months 1 hours
```

Closes #34969 from MaxGekk/fix-migr-guide-for-mixed-intervals.

Authored-by: Max Gekk <b91a34c712a29cd91cde3972985e114f1bb54d88@gmail.com>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

holdenk/spark,mahak/spark,zero323/spark,ueshin/apache-spark,srowen/spark,mahak/spark,srowen/spark,dongjoon-hyun/spark,holdenk/spark,nchammas/spark,zzcclp/spark,vinodkc/spark,zero323/spark,zero323/spark,witgo/spark,wangyum/spark,cloud-fan/spark,WeichenXu123/spark,gengliangwang/spark,hvanhovell/spark,wangyum/spark,cloud-fan/spark,witgo/spark,shaneknapp/spark,cloud-fan/spark,rednaxelafx/apache-spark,nchammas/spark,cloud-fan/spark,HyukjinKwon/spark,xuanyuanking/spark,shaneknapp/spark,HyukjinKwon/spark,wangyum/spark,gengliangwang/spark,HyukjinKwon/spark,JoshRosen/spark,zzcclp/spark,vinodkc/spark,hvanhovell/spark,zero323/spark,dongjoon-hyun/spark,nchammas/spark,ueshin/apache-spark,xuanyuanking/spark,xuanyuanking/spark,HyukjinKwon/spark,apache/spark,wangyum/spark,JoshRosen/spark,WeichenXu123/spark,cloud-fan/spark,apache/spark,zzcclp/spark,gengliangwang/spark,gengliangwang/spark,apache/spark,JoshRosen/spark,WeichenXu123/spark,hvanhovell/spark,srowen/spark,gengliangwang/spark,srowen/spark,vinodkc/spark,holdenk/spark,ueshin/apache-spark,WeichenXu123/spark,hvanhovell/spark,gengliangwang/spark,wangyum/spark,zzcclp/spark,HyukjinKwon/spark,zzcclp/spark,rednaxelafx/apache-spark,apache/spark,dongjoon-hyun/spark,JoshRosen/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,dongjoon-hyun/spark,HyukjinKwon/spark,WeichenXu123/spark,holdenk/spark,ueshin/apache-spark,holdenk/spark,mahak/spark,xuanyuanking/spark,nchammas/spark,nchammas/spark,HyukjinKwon/spark,vinodkc/spark,dongjoon-hyun/spark,JoshRosen/spark,witgo/spark,apache/spark,shaneknapp/spark,WeichenXu123/spark,mahak/spark,srowen/spark,witgo/spark,zero323/spark,witgo/spark,cloud-fan/spark,apache/spark,shaneknapp/spark,ueshin/apache-spark,xuanyuanking/spark,rednaxelafx/apache-spark,shaneknapp/spark,vinodkc/spark,mahak/spark,shaneknapp/spark,apache/spark,vinodkc/spark,zero323/spark,wangyum/spark,xuanyuanking/spark,srowen/spark,xuanyuanking/spark,nchammas/spark,vinodkc/spark,hvanhovell/spark,hvanhovell/spark,mahak/spark,ueshin/apache-spark,nchammas/spark,rednaxelafx/apache-spark,holdenk/spark,rednaxelafx/apache-spark,wangyum/spark,ueshin/apache-spark,shaneknapp/spark,WeichenXu123/spark,hvanhovell/spark,dongjoon-hyun/spark,mahak/spark,JoshRosen/spark,rednaxelafx/apache-spark,cloud-fan/spark,zzcclp/spark,witgo/spark,gengliangwang/spark,zzcclp/spark,srowen/spark,holdenk/spark,witgo/spark,zero323/spark,JoshRosen/spark