Fix leaflet.d.ts, TileLayerOptions.subdomains is flexibly typed

From the comments, subdomains can be a string or an array of strings. We
can't make any strong type inference from the interface.

Quote:
"Subdomains of the tile service. Can be passed in the form of one string (where
each letter is a subdomain name) or an array of strings."
Merge pull request #1025 from flxf/leafletany

Fix leaflet.d.ts, TileLayerOptions.subdomains is flexibly typed