[dcl.contract.res] Add period to end of sentence

Author: Andreas-Krug

source/declarations.tex

@@ -4719,7 +4719,7 @@
 the object or reference returned by
 invocation of that function.
 The type of a result binding
-is the return type of its associated function
+is the return type of its associated function.
 The optional \grammarterm{attribute-specifier-seq}
 of the \grammarterm{attributed-identifier}
 in the \grammarterm{result-name-introducer}