Remove unnecessary regex lookarounds in pagination docs

[BeksOmega]

Why:

Closes: N/A

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

Lookarounds aren't available in all regex parsers. And in this case they're unnecessary. Remove them to make it easier for folks to work with pagination in their own projects.

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 a 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.

[github-actions]

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Content directory changes

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Source	Preview	Production	What Changed
rest/using-the-rest-api/using-pagination-in-the-rest-api.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server