Fix RBS syntax error: void in union type on Readable#rewind

RBS 4.0 rejects `void` in union types at the parser level. The
`(Integer | void)` return type on `Readable#rewind` was always
semantically invalid and now causes a hard parse error.

Change to `void`, consistent with CompositeIO#rewind and the
YARD @return annotation. Rewind is called for its side effect.

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: () -> void
     end
 
     class Part