fix markup

Author: gfldex

doc/Type/Nil.pod6

@@ -36,7 +36,7 @@ that use a bare C<return> statement.
 In a list, C<Nil> takes the space of one value.  Iterating a C<Nil>
 behaves like iteration of any non-iterable value, producing a sequence
 of one C<Nil>. (When you need the other meaning, the special value
-C<Empty|/type/Slip#Empty> is available to take no spaces when inserted into
+L<Empty|/type/Slip#Empty> is available to take no spaces when inserted into
 list, and to return no values when iterated.)
 
     (1, Nil, 3).elems.say;      # 3