Update spec

Author: jgm

test/spec.txt

@@ -2894,6 +2894,17 @@ The link destination may not be omitted:
 .
 <p>[foo]:</p>
 <p>[foo]</p>
+````````````````````````````````
+
+ However, an empty link destination may be specified using
+ angle brackets:
+
+```````````````````````````````` example
+[foo]: <>
+
+[foo]
+.
+<p><a href="">foo</a></p>
 ````````````````````````````````
 
 The title must be separated from the link destination by