[SPARK-10760] [SPARKR] SparkR glm: the documentation in examples - family argument is missing

Hi everyone,

Since the family argument is required for the glm function, the execution of:

model <- glm(Sepal_Length ~ Sepal_Width, df)

is failing.

I've fixed the documentation by adding the family argument and also added the summay(model) which will show the coefficients for the model.

Thanks,
Narine

Author: Narine Kokhlikyan <narine.kokhlikyan@gmail.com>

Closes #8870 from NarineK/sparkrml.