Keypoints and objectives don't match for Exploring Data Frames episode

[sstevens2]

The Exploring Data Frames episode has an objective and keypoints that aren't' covered.

Specifically the objective objectives...

• Remove rows with NA values.
  and the keypoints...
• Remove rows from a data frame.
• Use na.omit() to remove rows from a data frame with NA values.

Also this one doesn't use typeof() - shoudl be class() instead

• Use str(), nrow(), ncol(), dim(), colnames(), rownames(), head(), and typeof() to understand the structure of a data frame.