Fix grammar in sponsor pitch: hyphenate well-maintained, add comma

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclark4life

index.html

@@ -119,7 +119,7 @@ <h2><i class="fas fa-heart" aria-hidden="true"></i> Support Pillow's Future</h2>
                             Pillow is free, open source software maintained by volunteers who review pull requests,
                             fix bugs, write documentation, and ship regular releases &mdash; all in their spare time.
                             Millions of projects depend on Pillow every day. If Pillow powers your product or
-                            business, consider sponsoring us to help keep it secure, fast and well maintained
+                            business, consider sponsoring us to help keep it secure, fast, and well-maintained
                             for everyone.
                         </p>
                     </div>