Fix markdown syntax in matrix.md

[couet]

fix markdown

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos7-multicore/default, ROOT-fedora29/python3, ROOT-fedora30/cxx14, ROOT-fedora31/noimt, ROOT-ubuntu16/nortcxxmod, mac1015/cxx17, windows10/cxx14
How to customize builds

[lmoneta]

Looks good, thank you Olivier for the fixes !

[phsft-bot]

Build failed on ROOT-performance-centos7-multicore/default.
Running on olhswep22.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See cdash.
See console output.

Errors:

• [2020-05-06T14:45:23.663Z] 646/2022 Test [TDF] minor fixes #370: tutorial-dataframe-df103_NanoAODHiggsAnalysis .....................................................***Failed Error regular expression found in output. Regex=[: error:] 4.20 sec

Failing tests:

• projectroot.runtutorials.tutorial_dataframe_df103_NanoAODHiggsAnalysis

[phsft-bot]

Build failed on ROOT-ubuntu16/nortcxxmod.
Running on sft-ubuntu-1604-4.cern.ch:/build/workspace/root-pullrequests-build
See cdash.
See console output.

Errors:

• [2020-05-06T15:01:47.181Z] CMake Error at /mnt/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:914 (message):

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft10.dyndns.cern.ch:/build/workspace/root-pullrequests-build
See cdash.
See console output.

Errors:

• [2020-05-06T17:20:32.613Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:914 (message):