Update code examples in README

[spacesailor24]

In README:

• Renames OptimismFeeEstimationPlugin to OptimismPlugin
• Adds plugin registration step to estimateFees code example

[codecov]

Codecov Report

Merging #6873 (fdab6ca) into develop (4bb2e6c) will increase coverage by 5.88%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6873      +/-   ##
===========================================
+ Coverage    44.81%   50.69%   +5.88%     
===========================================
  Files          376      314      -62     
  Lines        29535    23665    -5870     
  Branches      1692        0    -1692     
===========================================
- Hits         13235    11997    -1238     
+ Misses       14858    10621    -4237     
+ Partials      1442     1047     -395     

Flag	Coverage Δ
bedrock-go-tests	47.80% <ø> (-0.03%)	⬇️
cannon-go-tests	64.18% <ø> (ø)
chain-mon-tests	?
common-ts-tests	?
contracts-bedrock-tests	∅ <ø> (∅)
contracts-ts-tests	?
core-utils-tests	?
sdk-next-tests	?
sdk-tests	?

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see 64 files with indirect coverage changes

[mergify]

This PR has been added to the merge queue, and will be merged soon.

[mergify]

This PR is next in line to be merged, and will be merged as soon as checks pass.