Fix RBS syntax error

The newly-released RBS 4.0 rejects `void` in union types at the parser
level. The `(Integer | void)` return type on `Readable#rewind` now
causes a hard parse error.

Changed to `Integer`, matching IO#rewind and StringIO#rewind.

Author: hakanensari

sig/http.rbs

@@ -1272,7 +1272,7 @@ module HTTP
       def size: () -> Integer
 
       # Rewinds the IO to the beginning
-      def rewind: () -> (Integer | void)
+      def rewind: () -> Integer
     end
 
     class Part