Fix doc error for ExactSizeIterator

Fix doc error for ExactSizeIterator

The code example in the trait documentation of ExactSizeIterator
has an incorrect implementation of the len method that does not return
the number of times the example iterator 'Counter' will iterate. This
may confuse readers of the docs as the example code will compile but
doesn't uphold the trait's contract.

This is easily fixed by modifying the implementation of len and changing
the assert statement to actually assert the correct behaviour. I also
slightly modified a code comment to better reflect what the method
returns.