docs: fix typo from /decision to /decisions

[emmanuelgautier]

This PR fix documentation about the /decisions API endpoint.

Related issue(s)

No related issue.

Checklist

• I have read the contributing guidelines.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #1036 (d6f3797) into master (f363b3a) will not change coverage.
The diff coverage is n/a.

❗ Current head d6f3797 differs from pull request most recent head 583e7b4. Consider uploading reports for the commit 583e7b4 to get more accurate results

@@           Coverage Diff           @@
##           master    #1036   +/-   ##
=======================================
  Coverage   78.70%   78.70%           
=======================================
  Files          83       83           
  Lines        3869     3869           
=======================================
  Hits         3045     3045           
  Misses        554      554           
  Partials      270      270           

Impacted Files	Coverage Δ
api/decision.go	95.55% <ø> (ø)

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