Fix typo in github-username-policy.md (#25398)

Co-authored-by: Courtney Wilson <[email protected]>

content/site-policy/other-site-policies/github-username-policy.md

@@ -18,7 +18,7 @@ GitHub account names are available on a first-come, first-served basis, and are
 
 Keep in mind that not all activity on GitHub is publicly visible; accounts with no visible activity may be in active use.
 
-If the username you want has already been claimed, consider other names or unique variations. Using a number, hyphen, or an alternative spelling might help you identify a desirable username still available.
+If the username you want has already been claimed, consider other names or unique variations. Using a number, hyphen, or an alternative spelling might help you identify a desirable username that's still available.
 
 ## Trademark Policy