Update link in Hex metadata

Makefile

@@ -44,7 +44,7 @@ define HEX_TARBALL_EXTRA_METADATA
 #{
 	licenses => [<<"ISC">>],
 	links => #{
-		<<"Function reference">> => <<"https://ninenines.eu/docs/en/cowlib/2.10/manual/">>,
+		<<"Function reference">> => <<"https://ninenines.eu/docs/en/cowlib/2.11/manual/">>,
 		<<"GitHub">> => <<"https://github.com/ninenines/cowlib">>,
 		<<"Sponsor">> => <<"https://github.com/sponsors/essen">>
 	}