plural pullRequests in dependencyOverrides config suggestion

[samspills]

When scala-steward opens PRs it includes a little snippet to explain how to override the dependency config, and the suggestion looks like this

dependencyOverrides = [{
  pullRequest = { frequency = "@monthly" },
  dependency = { groupId = "com.blerf.dependency", artifactId = "dependency-client" }
}]

I tried to use this suggestion and got a parse error:

Attempt to decode value on failed cursor: DownField(pullRequests),DownArray,DownField(dependencyOverrides)

This PR updates pullRequest (singular) to pullRequests (plural)

[exoego]

Thanks!
Right, it should be prural. https://github.com/scala-steward-org/scala-steward/blob/main/docs/repo-specific-configuration.md

[exoego]

@samspills Could you also update the tests?

[samspills]

@exoego Just updated them!

[codecov]

Codecov Report

Merging #2610 (075fbbf) into main (bae7271) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2610   +/-   ##
=======================================
  Coverage   81.10%   81.10%           
=======================================
  Files         144      144           
  Lines        2540     2540           
  Branches       48       48           
=======================================
  Hits         2060     2060           
  Misses        480      480           

Impacted Files	Coverage Δ
...scalasteward/core/repoconfig/GroupRepoConfig.scala	75.00% <ø> (ø)

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