Describe 'user:email' as a required scope in the scope example

[Gyvastis]

As described in https://github.com/thephpleague/oauth2-github/issues/22

[codecov[bot]]

Codecov Report

Merging #23 (7efcbd0) into main (90fa802) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##                main       #23   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        26        26           
===========================================
  Files              3         3           
  Lines             52        52           
===========================================
  Hits              52        52           

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[shadowhand]

Thank you.