[BUG][GO] Replace references to sw package with {{packageName}} in README template

[joshraker]

A few years ago the examples in the README template for the Go client were updated to use the {{packageName}} provided to the generator instead of using sw as the hard-coded package name. However, not every reference to sw was replaced which caused me a good deal of confusion when trying to understand how to use the generated client. This PR updates the references to sw that were missed previously.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[joshraker]

Go Technical Committee: @antihax, @grokify, @kemokemo, @jirikuncar, @ph4r5h4d, @lwj5

[wing328]

lgtm. thanks for the PR.