fix comment describing the required cache interface

cache must respond to #del, not #keys

lib/generators/geocoder/config/templates/initializer.rb

@@ -7,7 +7,7 @@ Geocoder.configure(
   # http_proxy: nil,            # HTTP proxy server (user:pass@host:port)
   # https_proxy: nil,           # HTTPS proxy server (user:pass@host:port)
   # api_key: nil,               # API key for geocoding service
-  # cache: nil,                 # cache object (must respond to #[], #[]=, and #keys)
+  # cache: nil,                 # cache object (must respond to #[], #[]=, and #del)
   # cache_prefix: 'geocoder:',  # prefix (string) to use for all cache keys
 
   # Exceptions that should not be rescued by default