Fix /monitoring link in index.html.md

[mauricioschneider]

There's a type in the link to /monitoring on the Explore Fly.io by features section.

Summary of changes

Fix the link from /moitoring/ to /monitoring/

[shinebayar-g]

It's a duplicate of #1938. But they already pushed to the master a26ed67

[mauricioschneider]

@shinebayar-g good catch!