fix context-free grammar notation

Author: twof

spec/Appendix B -- Grammar Summary.md

@@ -156,10 +156,12 @@ Selection :
 - FragmentSpread
 - InlineFragment
 
-Field : Alias? Name Arguments? Directives? SelectionSet?
+Field : Alias? Name Arguments? Nullability? Directives? SelectionSet?
 
 Alias : Name :
 
+Nullability : (`?`|`!`)?
+
 Arguments[Const] : ( Argument[?Const]+ )
 
 Argument[Const] : Name : Value[?Const]

spec/Section 2 -- Language.md

@@ -517,7 +517,7 @@ which returns the result:
 
 ## Nullability
 
-Name Nullability?
+Nullability : (`?`|`!`)?
 
 Fields can have their nullability designated with either a `!` to indicate that a
 field should be `Non-Nullable` or a `?` to indicate that a field should be