[MRG] Update docstring to include 'a' parameter

[caph1993]

Added parameter description for 'a' in the docstring.

Types of changes

The changes are only in the documentation of the function averaged_sgd_entropic_transport.

Motivation and context / Related issue

Parameter a is missing in the documentation.

How has this been tested (if it applies)

Does not apply.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (2403a4c) to head (a1fbdd2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #769   +/-   ##
=======================================
  Coverage   97.15%   97.15%           
=======================================
  Files         107      107           
  Lines       21877    21877           
=======================================
  Hits        21254    21254           
  Misses        623      623           

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