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.

Change to `void`, consistent with CompositeIO#rewind and the YARD
@return annotation.

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