workflows/copilot-instructions.md: Misspelled term

[VimalaMoger]

Why:

This PR for the term 'external' which is not correctly written in the text. I corrected the word which help convey clear message to users

What's being changed (if available, include any code snippets, screenshots, or gifs):

under

Testing changes - section

For integration tests, we can use the mock server in src/tests/mocks/start-mock-server.ts to mock exteneral requests.

Updated version:
For integration tests, we can use the mock server in src/tests/mocks/start-mock-server.ts to mock external requests.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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[github-actions]

👋 Hey there spelunker. It looks like you've modified some files that we can't accept as contributions:

• .github/copilot-instructions.md

You'll need to revert all of the files you changed that match that list using GitHub Desktop or git checkout origin/main <file name>. Once you get those files reverted, we can continue with the review process.

The complete list of files we can't accept are:

• .devcontainer/**
• .github/**
• data/reusables/rai/**
• Dockerfile*
• src/**
• package*.json
• content/actions/how-tos/security-for-github-actions/security-hardening-your-deployments/**

We also can't accept contributions to files in the content directory with frontmatter type: rai or contentType: rai.