[SPARK-20208][DOCS][FOLLOW-UP] Add FP-Growth to SparkR programming guide

## What changes were proposed in this pull request?

Add `spark.fpGrowth` to SparkR programming guide.

## How was this patch tested?

Manual tests.

Author: zero323 <[email protected]>

Closes #17775 from zero323/SPARK-20208-FOLLOW-UP.

Author: zero323

docs/sparkr.md

@@ -504,6 +504,10 @@ SparkR supports the following machine learning algorithms currently:
 
 * [`spark.als`](api/R/spark.als.html): [`Alternating Least Squares (ALS)`](ml-collaborative-filtering.html#collaborative-filtering)
 
+#### Frequent Pattern Mining
+
+* [`spark.fpGrowth`](api/R/spark.fpGrowth.html) : [`FP-growth`](ml-frequent-pattern-mining.html#fp-growth)
+
 #### Statistics
 
 * [`spark.kstest`](api/R/spark.kstest.html): `Kolmogorov-Smirnov Test`