[MINOR] [ML] fix wrong annotation of RFormula.formula

fix wrong annotation of RFormula.formula

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #7470 from yanboliang/RFormula and squashes the following commits:

61f1919 [Yanbo Liang] fix wrong annotation

mllib/src/main/scala/org/apache/spark/ml/feature/RFormula.scala

@@ -42,7 +42,7 @@ class RFormula(override val uid: String)
 
   /**
    * R formula parameter. The formula is provided in string form.
-   * @group setParam
+   * @group param
    */
   val formula: Param[String] = new Param(this, "formula", "R model formula")