doc/go: remove platform from example

this should only be set if the package doesn't work with the default platforms

doc/languages-frameworks/go.section.md

···
     homepage = "https://github.com/knqyf263/pet";
     license = licenses.mit;
     maintainers = with maintainers; [ kalbasit ];
-    platforms = platforms.linux ++ platforms.darwin;
   };
 }
 ```