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

@@ -69,7 +69,7 @@ Marks the field, argument, input field or enum value as deprecated.
 """
 directive @deprecated(
   """
-  The reason for the deprecation
+  The reason for the deprecation.
   """
   reason: String = "No longer supported"
 ) on FIELD_DEFINITION | ARGUMENT_DEFINITION | ENUM_VALUE | INPUT_FIELD_DEFINITION