docs(vrl): update documentation for parse_nginx_log with the main log format

[virtualtam]

Summary

This PR updates the documentation for the parse_nginx_log VRL remap function with an example for parsing a log line formatted using the main format (see VRL issue and PR referenced below for more information).

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

I have built the website documentation locally with:

$ cd rust-docs
$ make docs

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Please read our Vector contributor resources.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run dd-rust-license-tool write to regenerate the license inventory and commit the changes (if any). More details here.

References

• VRL issue: VRL: add main log format for parse_nginx_log vrl#1201
• VRL PR: feat(stdlib): Add main log format to parse_nginx_log function vrl#1202

Notes for reviewers

• Looking at the VRL and Vector contributing guides, I am not sure whether this PR requires adding a changelog fragment; if so, please suggest the appropriate format for the changelog entry 📖