Simplify rewind return type signature

The (Integer | void) union is redundant since void already permits any
return value.

Author: sferik

sig/http/form_data/readable.rbs

@@ -13,7 +13,7 @@ module HTTP
       def size: () -> Integer
 
       # Rewinds the IO to the beginning
-      def rewind: () -> (Integer | void)
+      def rewind: () -> void
     end
   end
 end