Should pass spark context to save/load

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4816 from jkbradley/ml-io-doc-fix and squashes the following commits:

83d369d [Joseph K. Bradley] added comment to save,load parts of ML guide examples
2841170 [Joseph K. Bradley] Fixed save,load calls in ML guide examples