Correct `EarthLocation.of_address()` documentation
Correct `EarthLocation.of_address()` documentation

`EarthLocation.of_address()` used to use Google Maps API, but when
Google started requiring an API key #7918 made the OpenStreetMap
Nominatim service the default. However, the documentation was not
updated until now.

pllim/astropy,pllim/astropy,astropy/astropy,lpsinger/astropy,mhvk/astropy,pllim/astropy,mhvk/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,larrybradley/astropy,lpsinger/astropy,astropy/astropy,larrybradley/astropy,pllim/astropy,mhvk/astropy,lpsinger/astropy,astropy/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy