[SPARK-20026][DOC][SPARKR] Add Tweedie example for SparkR in programming guide

## What changes were proposed in this pull request?
Add Tweedie example for SparkR in programming guide.
The doc was already updated in #17103.

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #17553 from actuaryzhang/programGuide.

examples/src/main/r/ml/glm.R

@@ -56,6 +56,15 @@ summary(binomialGLM)
 # Prediction
 binomialPredictions <- predict(binomialGLM, binomialTestDF)
 head(binomialPredictions)
+
+# Fit a generalized linear model of family "tweedie" with spark.glm
+training3 <- read.df("data/mllib/sample_multiclass_classification_data.txt", source = "libsvm")
+tweedieDF <- transform(training3, label = training3$label * exp(randn(10)))
+tweedieGLM <- spark.glm(tweedieDF, label ~ features, family = "tweedie",
+                        var.power = 1.2, link.power = 0)
+
+# Model summary
+summary(tweedieGLM)
 # $example off$
 
 sparkR.session.stop()