Fix typo (s/containement/containment/)

Author: dholbert

css-contain-1/Overview.bs

@@ -324,7 +324,7 @@ Paint Containment</h3>
 
 <h2 id="becoming-formatting-context">Becoming a formatting context root</h2>
 
-In some some types of containement,
+In some some types of containment,
 a box may need to <dfn export lt='become a formatting context root|becomes a formatting context root|becoming a formatting context root'>become a <a>formatting context</a> root</dfn>.
 If the box already establishes a <a>formatting context</a> of any kind other than an <a>inline formatting context</a>,
 this condition is satisfied.