remove ? from grammar spec

Author: twof

spec/Appendix B -- Grammar Summary.md

@@ -160,7 +160,7 @@ Field : Alias? Name Arguments? Nullability? Directives? SelectionSet?
 
 Alias : Name :
 
-Nullability : (`?`|`!`)?
+Nullability : (`?`|`!`)
 
 Arguments[Const] : ( Argument[?Const]+ )
 

spec/Section 2 -- Language.md

@@ -517,7 +517,7 @@ which returns the result:
 
 ## Nullability
 
-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