Fix typo

index.bs

@@ -1877,7 +1877,7 @@ To <dfn>check if cookie-based debugging is allowed</dfn> given a
 
 To obtain the <dfn export for=node>context origin</dfn> of a [=node=] |node|:
 
-1. If applicable for [=implemenation-defined=] use cases, return the embedded origin.
+1. If applicable for [=implementation-defined=] use cases, return the embedded origin.
 1. Otherwise, return |node|'s [=node navigable=]'s [=navigable/top-level traversable=]'s
     [=navigable/active document=]'s [=origin=].