[3786] remove "ext/" from reference to the boost license file

because that directory name doesn't exist with the installed version
of kea.

This is trivial and I commit to master without review.

COPYING

@@ -16,4 +16,4 @@ PERFORMANCE OF THIS SOFTWARE.
 
 The ext/asio and ext/coroutine code is externally maintained and
 distributed under the Boost Software License, Version 1.0.
-(See accompanying file ext/LICENSE_1_0.txt.)
+(See its accompanying file LICENSE_1_0.txt.)