add 'USATopo' to API reference

[jgravois]

we're looking for help from someone to add new information to the API reference to mention the new basemap key that @mattficke added in Esri/esri-leaflet#758

1. check out our 'getting started'
2. the source code for L.esri.basemapLayer can be found here
3. commit your changes and submit a pull request to compare your own branch with upcoming

note: we'll use upcoming as the branch in this repository to store the documentation change because the new basemap won't be available to end users until after we tag our next release of Esri Leaflet.

i'm happy to answer questions if any of this is unclear.

[jgravois]

94f225e