godoc: fix codewalk handler
godoc: fix codewalk handler

For URLs ending with /, the handler did not work, trying to append
".xml" to the path.

For instance, the "Share Memory by Communicating" returned the
following error:

        open /Users/francisco.souza/lib/go/doc/codewalk/sharemem/.xml: no such file or directory

R=adg, minux.ma
CC=golang-dev
http://codereview.appspot.com/5797065

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,bryanxu/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,Triskite/willstone-goclone,webfd/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,d0f/go-zh