doc: fix typo "heirarchy"

Change-Id: Iae2bf44ec15975f440d026fd52fcccfbd9c598d9
Reviewed-on: https://go-review.googlesource.com/17740Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go_faq.html

@@ -885,7 +885,7 @@ Go separates the notion of what a
 type does—its methods—from the type's implementation.
 If two methods return different types, they are not doing the same thing.
 Programmers who want covariant result types are often trying to
-express a type heirarchy through interfaces.
+express a type hierarchy through interfaces.
 In Go it's more natural to have a clean separation between interface
 and implementation.
 </p>