Small grammar change on "About geo-replication" page

[austenstone]

Why:

Incorrect grammar.

What's being changed:

are limited by the slowest replica should be is limited by the slowest replica

This means that the performance of all writes is limited by the slowest replica, although new geo-replicas can seed the majority of their data from existing co-located geo-replicas, rather than from the primary.

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Source	Staging	Production	What Changed
content/admin/enterprise-management/configuring-high-availability/about-geo-replication.md	Modified	Original

[ramyaparimi]

@astone2014
Thanks so much for opening a PR with this fix! I'll update the branch and get this merged once tests are passing 🍏

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