Fix zoom levels for the MapQuest Street layer

A typo in 65b8e0f91576e521606d906ecdfa9dffd2159dcb introduced this bug.
Before that, 18 zoom levels were used. Now we're using 19, which appears
to be available with full world coverage.
Merge pull request #2978 from ahocevar/mapquest-zoom

Fix zoom levels for the MapQuest Street layer