[ruby/io-console] [DOC] IO::console.getpass usage example

There were no clear example of this very useful method's usage anywhere
in the IO or IO::Console docs, which was a shame.

https://github.com/ruby/io-console/commit/4d324586a8

ext/io/console/console.c

@@ -1578,6 +1578,12 @@ str_chomp(VALUE str)
  * see String#chomp!.
  *
  * You must require 'io/console' to use this method.
+ * 
+ *    require 'io/console'
+ *    IO::console.getpass("Enter password: ")
+ *    Enter password: 
+ *    # => "mypassword"
+ * 
  */
 static VALUE
 console_getpass(int argc, VALUE *argv, VALUE io)