fixes errors in the multi-file building example

[Scott-Fischer]

Changes

For anyone following along with the reference/guide docs line-by-line, they would encounter a syntax and reference error in the multi-file building example code.

1. The syntax error is thrown after the return statement (missing comma).

2. Once that is fixed, the "codeToCompile" variable is undefined.

This should fix both errors and allow readers to run the example plugin without any errors.

Testing

No tests added because it's a change to the documentation via a markdown file

Docs

No new docs added, just an update to www/_template/guides/plugins.md

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[FredKSchott]

ah, great catch. Thanks for finding/fixing!