Fix doctest in min_spanning_tree()

8 jobs for master
Status Name Job ID Coverage
  Test
passed job_clang_amd64 #554
amd64

02:58:04

passed job_clang_amd64_nosh #558
amd64

03:00:50

passed job_clang_py2_amd64 #556
amd64

02:58:22

passed job_gcc_386 #551
i386

03:04:42

passed job_gcc_amd64 #553
amd64

02:16:17

passed job_gcc_amd64_nosh #557
amd64

02:16:31

passed job_gcc_py2_386 #552
i386

03:00:33

passed job_gcc_py2_amd64 #555
amd64

02:15:50