[SPARK-24423][FOLLOW-UP][SQL] Fix error example

## What changes were proposed in this pull request?
![image](https://user-images.githubusercontent.com/5399861/49172173-42ad9800-f37b-11e8-8135-7adc323357ae.png)
It will throw:
```
requirement failed: When reading JDBC data sources, users need to specify all or none for the following options: 'partitionColumn', 'lowerBound', 'upperBound', and 'numPartitions'
```
and
```
User-defined partition column subq.c1 not found in the JDBC relation ...
```

This PR fix this error example.

## How was this patch tested?

manual tests

Closes #23170 from wangyum/SPARK-24499.

Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
(cherry picked from commit 06a3b6a)
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: wangyum

docs/sql-data-sources-jdbc.md

@@ -64,9 +64,9 @@ the following case-insensitive options:
             Example:<br>
             <code>
                spark.read.format("jdbc")<br>
-               &nbsp&nbsp .option("dbtable", "(select c1, c2 from t1) as subq")<br>
-               &nbsp&nbsp .option("partitionColumn", "subq.c1"<br>
-               &nbsp&nbsp .load()
+                 .option("url", jdbcUrl)<br>
+                 .option("query", "select c1, c2 from t1")<br>
+                 .load()
             </code></li>
       </ol>
     </td>

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCOptions.scala

@@ -137,9 +137,13 @@ class JDBCOptions(
        |the partition columns using the supplied subquery alias to resolve any ambiguity.
        |Example :
        |spark.read.format("jdbc")
-       |        .option("dbtable", "(select c1, c2 from t1) as subq")
-       |        .option("partitionColumn", "subq.c1"
-       |        .load()
+       |  .option("url", jdbcUrl)
+       |  .option("dbtable", "(select c1, c2 from t1) as subq")
+       |  .option("partitionColumn", "c1")
+       |  .option("lowerBound", "1")
+       |  .option("upperBound", "100")
+       |  .option("numPartitions", "3")
+       |  .load()
      """.stripMargin
   )
 

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -1348,9 +1348,13 @@ class JDBCSuite extends QueryTest
          |the partition columns using the supplied subquery alias to resolve any ambiguity.
          |Example :
          |spark.read.format("jdbc")
-         |        .option("dbtable", "(select c1, c2 from t1) as subq")
-         |        .option("partitionColumn", "subq.c1"
-         |        .load()
+         |  .option("url", jdbcUrl)
+         |  .option("dbtable", "(select c1, c2 from t1) as subq")
+         |  .option("partitionColumn", "c1")
+         |  .option("lowerBound", "1")
+         |  .option("upperBound", "100")
+         |  .option("numPartitions", "3")
+         |  .load()
      """.stripMargin
     val e5 = intercept[RuntimeException] {
       sql(