fix: replace incorrect branch key with correct branches key in releaserc file

[MichaelRoytman]

The release GitHub action on the previous pull request failed with the following error.

SemanticReleaseError: The release branches are invalid in the `branches` configuration.
    at default (file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/lib/get-error.js:6:10)
    at file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/lib/branches/index.js:45:19
    at Array.reduce (<anonymous>)
    at default (file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/lib/branches/index.js:35:[46](https://github.com/edx/frontend-lib-learning-assistant/actions/runs/5754323807/job/15599367963#step:7:47))
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async run (file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/index.js:68:22)
    at async Module.default (file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/index.js:275:22)
    at async default (file:///home/runner/work/frontend-lib-learning-assistant/frontend-lib-learning-assistant/node_modules/semantic-release/cli.js:55:5) {
  code: 'ERELEASEBRANCHES',
  details: 'A minimum of 1 and a maximum of 3 release branches are required in the [branches configuration](https://github.com/semantic-release/semantic-release/blob/master/docs/usage/configuration.md#branches).\n' +
    '\n' +
    'This may occur if your repository does not have a release branch, such as `master`.\n' +
    '\n' +
    'Your configuration for the problematic branches is `[]`.',
  semanticRelease: true
}

I copied the .releaserc file from frontend-lib-special-exams and forgot to rename branch to branches. branches is used by semantic-release to determine which Git branches to release; see here. This is necessary because we release on a non-standard main branch - not master.

[codecov]

Codecov Report

Merging #2 (2cec826) into main (987a3c7) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main      #2   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          2       2           
  Lines          7       7           
=====================================
  Misses         7       7           

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