update boost.asio link in documentation, minor cleanup of tutorial examples

docs/manual.rst

@@ -109,7 +109,7 @@ with an associated port.
 
 For documentation on these types, please refer to the `asio documentation`_.
 
-.. _`asio documentation`: http://asio.sourceforge.net/asio-0.3.8/doc/asio/reference.html
+.. _`asio documentation`: http://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio.html
 
 exceptions
 ==========

examples/bt-get.cpp

@@ -41,7 +41,6 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <libtorrent/magnet_uri.hpp>
 #include <libtorrent/error_code.hpp>
 
-namespace lt = libtorrent;
 int main(int argc, char const* argv[])
 {
 	if (argc != 2) {

examples/bt-get2.cpp

@@ -44,7 +44,6 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <libtorrent/magnet_uri.hpp>
 #include <libtorrent/error_code.hpp>
 
-namespace lt = libtorrent;
 using clk = std::chrono::steady_clock;
 
 // return the name of a torrent status enum