fix british spelling

Author: magicmark

spec/Section 3 -- Type System.md

@@ -2338,7 +2338,7 @@ DirectiveArgumentCoordinate : @ Name ( Name : )
 
 **Resolving Meta-fields**
 
-Resolving the _schema coordinate_ of a meta-field is undefined behaviour, since
+Resolving the _schema coordinate_ of a meta-field is undefined behavior, since
 these fields may be implemented in such a way that makes it impossible to return
 a unique value specific to that instance of the meta-field.