Fix typo in controllers.md

[amirc]

Description of what you did:

The default SingleType service doesn't have an findOne() function only find().

Closes #7718

[codecov]

Codecov Report

Merging #7721 into documentation will increase coverage by 8.02%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           documentation    #7721      +/-   ##
=================================================
+ Coverage          19.14%   27.16%   +8.02%     
=================================================
  Files                855     1163     +308     
  Lines              11933    15518    +3585     
  Branches            1898     2410     +512     
=================================================
+ Hits                2284     4215    +1931     
- Misses              8100     9535    +1435     
- Partials            1549     1768     +219     

Flag	Coverage Δ
#front	19.31% <ø> (+4.82%)	⬆️
#unit	53.85% <ø> (+13.92%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/strapi-plugin-email/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...api-plugin-users-permissions/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...n/src/containers/NotificationProvider/constants.js	0.00% <0.00%> (-100.00%)	⬇️
...i-admin/admin/src/containers/SettingsPage/index.js	0.00% <0.00%> (-66.67%)	⬇️
...trapi-admin/admin/src/containers/LeftMenu/index.js	5.00% <0.00%> (-61.67%)	⬇️
...n/admin/src/containers/LanguageProvider/reducer.js	0.00% <0.00%> (-61.54%)	⬇️
...in/admin/src/containers/Webhooks/ListView/index.js	0.00% <0.00%> (-42.11%)	⬇️
...in/admin/src/containers/Webhooks/EditView/index.js	0.00% <0.00%> (-38.40%)	⬇️
packages/strapi/lib/services/metrics/index.js	65.00% <0.00%> (-35.00%)	⬇️
...admin/admin/src/components/ContainerFluid/index.js	66.66% <0.00%> (-33.34%)	⬇️
... and 746 more

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[alexandrebodin]

Nice catch !