Update about-githubs-ip-addresses.md

[LinuxOnTheDesktop]

I have corrected an ungrammatical use of 'however': that word is not a conjunction, and unclarity can result from using it as if it were a conjunction.

Why:

Closes:

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

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. [I have 'ticked' this, but it is not applicable.]
• The changes in this PR meet the docs fundamentals that are required for all content. [Surely grammatical fixes are allowed.]
• All CI checks are passing and the changes look good in the review environment. [Surely these checks - which have yet to be run - will pass.]

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Source	Review	Production	What Changed
authentication/keeping-your-account-and-data-secure/about-githubs-ip-addresses.md	fpt ghec	fpt ghec

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[subatoi]

@LinuxOnTheDesktop Thanks for spotting that: I've changed this to the word "but" because that's the slightly better edit under the circumstances. I'll get this merged for you now—thanks for your interest in the GitHub Docs!

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