fix typo in IO#write_nonblock example [ci skip]

This example explains when writing a value greater than 65536,
so the value specify must be greater than 65536.

This seems to be wrong in cee7f69

Patch by: Yuji Yaginuma <yuuji.yaginuma@gmail.com>
[Fix rubyGH-1496]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57018 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

prelude.rb

@@ -99,7 +99,7 @@ def read_nonblock(len, buf = nil, exception: true)
   #
   #   # write_nonblock writes only 65536 bytes and return 65536.
   #   # (The pipe size is 65536 bytes on this environment.)
-  #   s = "a"  #100000
+  #   s = "a" * 100000
   #   p w.write_nonblock(s)     #=> 65536
   #
   #   # write_nonblock cannot write a byte and raise EWOULDBLOCK (EAGAIN).