[SPARK-4972][MLlib] Updated the scala doc for lasso and ridge regression for the change of LeastSquaresGradient

[dbtsai]

In #SPARK-4907, we added factor of 2 into the LeastSquaresGradient. We updated the scala doc for lasso and ridge regression here.

[SparkQA]

Test build #24832 has started for PR 3808 at commit ec3c989.

• This patch merges cleanly.

[SparkQA]

Test build #24832 has finished for PR 3808 at commit ec3c989.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[mengxr]

LGTM. Merged into master. Thanks!