Fix Section 7 examples numbering in the list of examples

[lbdreyer]

When the new example Specifying formula_terms when a parametric coordinate variable has bounds was added, the numbering in the list of examples was not updated.

I also noticed, in the same section, that Examples 7.15 and 7.16 were missing from the list of examples

[dblodgett-usgs]

Thanks! It's been nearly three weeks with no comment and this is so clearly a defect fix, I'm going to go ahead and merge.

[lbdreyer]

Thanks for merging @dblodgett-usgs !