[SPARK-3953][SQL][Minor] Confusable variable name.

In SqlParser.scala, there is following code.

    case d ~ p ~ r ~ f ~ g ~ h ~ o ~ l  =>
      val base = r.getOrElse(NoRelation)
      val withFilter = f.map(f => Filter(f, base)).getOrElse(base)

In the code above, there are 2 variables which have same name "f" in near place.
One is receiver "f" and other is bound variable "f".

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2807 from sarutak/SPARK-3953 and squashes the following commits:

4957c32 [Kousuke Saruta] Improved variable name in SqlParser.scala