[SQL][DOCS] Remove wrong example from DataFrame.scala

In DataFrame.scala, there are examples like as follows.

```
 * // The following are equivalent:
 * peopleDf.filter($"age" > 15)
 * peopleDf.where($"age" > 15)
 * peopleDf($"age" > 15)
```

But, I think the last example doesn't work.

Author: Kousuke Saruta <[email protected]>

Closes #6977 from sarutak/fix-dataframe-example and squashes the following commits:

46efbd7 [Kousuke Saruta] Removed wrong example

Author: sarutak

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -682,7 +682,6 @@ class DataFrame private[sql](
    *   // The following are equivalent:
    *   peopleDf.filter($"age" > 15)
    *   peopleDf.where($"age" > 15)
-   *   peopleDf($"age" > 15)
    * }}}
    * @group dfops
    * @since 1.3.0
@@ -707,7 +706,6 @@ class DataFrame private[sql](
    *   // The following are equivalent:
    *   peopleDf.filter($"age" > 15)
    *   peopleDf.where($"age" > 15)
-   *   peopleDf($"age" > 15)
    * }}}
    * @group dfops
    * @since 1.3.0