guides/fix example heading for WTR and content as data snippet

[thescientist13]

Related Issue

Noticed when trying to follow this example for WTR <> Content as data integration, and copying the window.fetch snippet, the code snippet heading implied this should go in web-test-runner.config.js

which leads to this error if you do

➜  www.owenascode.dev git:(feature/issue-10-header-component) ✗ npm run test

> [email protected] test
> wtr

ReferenceError: window is not defined
    at file:///Users/owenbuckley/Workspace/github/www.owenascode.dev/web-test-runner.config.js:8:1
    at ModuleJob.run (node:internal/modules/esm/module_job:271:25)
    at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:547:26)
    at async importConfig (/Users/owenbuckley/Workspace/github/www.owenascode.dev/node_modules/.pnpm/@[email protected]/node_modules/@web/config-loader/src/importConfig.js:17:20)
    at async readFileConfig (/Users/owenbuckley/Workspace/github/www.owenascode.dev/node_modules/.pnpm/@[email protected]/node_modules/@web/test-runner/dist/config/readFileConfig.js:13:16)
    at async startTestRunner (/Users/owenbuckley/Workspace/github/www.owenascode.dev/node_modules/.pnpm/@[email protected]/node_modules/@web/test-runner/dist/startTestRunner.js:20:15)

Looking closer at the snippet, it becomes obvious the window.fetch snippet is supposed to go into the test case file itself.

Summary of Changes

1. Fixed the snippet heading to indicate this is for a test file (not the WTR config file 😅 )

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