Refactors pageTitled onlyOneH1 rule to maximumTwoH1 

Changing the onlyOneH1 rule to match the description of the rule as stated in the URL shown. 

http://oaa-accessibility.org/rule/17/

Renamed onlyOneH1 to maximumTwoH1, updated the message and length check accordingly.

Resolves globant-ui/arialinter#13