[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 <sarutak@oss.nttdata.co.jp>

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

46efbd7 [Kousuke Saruta] Removed wrong example