[RLlib]: Fix OPE docs

[Rohan138]

Signed-off-by: Rohan138 [email protected]

#27154 changed the tau parameter to polyak_coef, but I forgot to update the docs. This should also fix this post.

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• [X] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [X] I've run scripts/format.sh to lint the changes in this PR.
• [X] I've included any doc changes needed for https://docs.ray.io/en/master/.
• [X] I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • [ ] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(

[kouroshHakha]

@sven1977 please merge this.