grammar fixes
index.html

The fixes are really minor fixes, but they would have two "not-minor" effects:
* The strings need to be retranslated by all L10n teams
* We will probably rewrite this page entirely for fedora.next, see #248

This said, I think we shouldn't change things on features and using sites now, both are outdated and need fixes. Let's do them when we know concretely how we want to redesign them and what to display or promote there.