correct comma to period

Author: schultzdavid

doc/Type/Attribute.rakudoc

@@ -169,7 +169,7 @@ Marks as C<readonly> an attribute of a class that has the C<is rw> trait.
     # OUTPUT: «Cannot modify an immutable Str (copper)␤...»
     =end code
 
-The attributes of classes without the C<is rw> trait are already readonly by default,
+The attributes of classes without the C<is rw> trait are already readonly by default.
 Thus, in classes without the C<is rw> trait, the attribute trait C<is readonly> is redundant.
 
 =head2 X<trait is built|Traits,is built>