Update spec/Appendix C -- Built-in Definitions.md

Co-authored-by: Benoit 'BoD' Lubek <[email protected]>

Author: martinbonnin

spec/Appendix C -- Built-in Definitions.md

@@ -45,7 +45,7 @@ scalar ID
 
 ```graphql
 """
-Directs the executor to include this field or fragment only when the `if` argument is true
+Directs the executor to include this field or fragment only when the `if` argument is true.
 """
 directive @include(
   """