MLlib documentation fix

Synchronized mllib-optimization.md with Spark Scaladoc: removed reference to GradientDescent.runMiniBatchSGD method

This is a temporary fix to remove  a link from http://spark.apache.org/docs/latest/mllib-optimization.html to GradientDescent.runMiniBatchSGD which is not in the current online GradientDescent Scaladoc.
FIXME: revert this commit after GradientDescent Scaladoc is updated.
See images for details.

![mllib-docs-fix-1](https://cloud.githubusercontent.com/assets/1375501/3294410/ccf19bb8-f5a8-11e3-93f1-f593016209eb.png)
![mllib-docs-fix-2](https://cloud.githubusercontent.com/assets/1375501/3294411/d0b59a7e-f5a8-11e3-8fc8-329c177ef8c8.png)