Fix list issue of Scala API doc

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

@@ -47,18 +47,17 @@ private[feature] trait RFormulaBase extends HasFeaturesCol with HasLabelCol {
  * the R formula docs here: http://stat.ethz.ch/R-manual/R-patched/library/stats/html/formula.html
  *
  * The basic operators are:
- *
- * `~` separate target and terms
- * `+` concat terms, "+ 0" means removing intercept
- * `-` remove a term, "- 1" means removing intercept
- * `:` interaction (multiplication for numeric values, or binarized categorical values)
- * `.` all columns except target
+ *  - `~` separate target and terms
+ *  - `+` concat terms, "+ 0" means removing intercept
+ *  - `-` remove a term, "- 1" means removing intercept
+ *  - `:` interaction (multiplication for numeric values, or binarized categorical values)
+ *  - `.` all columns except target
  *
  * Suppose `a` and `b` are double columns, we use the following simple examples
  * to illustrate the effect of `RFormula`:
- * `y ~ a + b` means model `y ~ w0 + w1 * a + w2 * b` where `w0` is the intercept and `w1, w2`
+ *  - `y ~ a + b` means model `y ~ w0 + w1 * a + w2 * b` where `w0` is the intercept and `w1, w2`
  * are coefficients.
- * `y ~ a + b + a:b - 1` means model `y ~ w1 * a + w2 * b + w3 * a * b` where `w1, w2, w3`
+ *  - `y ~ a + b + a:b - 1` means model `y ~ w1 * a + w2 * b + w3 * a * b` where `w1, w2, w3`
  * are coefficients.
  *
  * RFormula produces a vector column of features and a double or string column of label.