[DOC] fixed the return value of IO#ready? [ci skip]

IO#ready? returns true or false only, since r50262(1baa57b003).

ext/io/wait/wait.c

@@ -95,10 +95,9 @@ io_nread(VALUE io)
 
 /*
  * call-seq:
- *   io.ready? -> true, false or nil
+ *   io.ready? -> true or false
  *
  * Returns true if input available without blocking, or false.
- * Returns nil if no information available.
  */
 
 static VALUE