[SPARKR][DOC] Document LinearSVC in R programming guide

## What changes were proposed in this pull request?

add link to svmLinear in the SparkR programming document.

## How was this patch tested?

Build doc manually and click the link to the document. It looks good.

Author: wangmiao1981 <wm624@hotmail.com>

Closes #17797 from wangmiao1981/doc.

docs/sparkr.md

@@ -482,6 +482,7 @@ SparkR supports the following machine learning algorithms currently:
 * [`spark.logit`](api/R/spark.logit.html): [`Logistic Regression`](ml-classification-regression.html#logistic-regression)
 * [`spark.mlp`](api/R/spark.mlp.html): [`Multilayer Perceptron (MLP)`](ml-classification-regression.html#multilayer-perceptron-classifier)
 * [`spark.naiveBayes`](api/R/spark.naiveBayes.html): [`Naive Bayes`](ml-classification-regression.html#naive-bayes)
+* [`spark.svmLinear`](api/R/spark.svmLinear.html): [`Linear Support Vector Machine`](ml-classification-regression.html#linear-support-vector-machine)
 
 #### Regression