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

@@ -65,7 +65,7 @@ directive @skip(
 ) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT
 
 """
-Marks the field, argument, input field or enum value as deprecated
+Marks the field, argument, input field or enum value as deprecated.
 """
 directive @deprecated(
   """