Fix missing link in Sensei plugin description.

[anaemnesis]

Proposed Changes

Ensures that:

Submit your course to Sensei LMS' public course showcase at https://senseilms.com/showcase/. Sensei LMS' course showcase allows you to reach new audiences to expand your course's visibility.

Links to https://senseilms.com/showcase/, whereas at present, it does not:

From https://wordpress.org/plugins/sensei-lms/:

Testing Instructions

1. Download branch
2. Validate markdown

This PR needs to be approved and merged here, and the changes copied over to the SVN repo for deployment there too, which I don't have access to.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #6853 (9b0597b) into trunk (c5e4e58) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              trunk    #6853   +/-   ##
=========================================
  Coverage     47.37%   47.37%           
  Complexity    10124    10124           
=========================================
  Files           499      499           
  Lines         35995    35995           
  Branches        283      283           
=========================================
  Hits          17054    17054           
  Misses        18729    18729           
  Partials        212      212           

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[donnapep]

Looks good. I think this one can likely wait to be deployed with our next release.