types: mention that TLV-TYPE 31 used to be allocated

Amends commit 9fba5f4c46b97eeaf22f5e2c33e6368a51272255

Change-Id: If43f60b9e9077e95ca6bd79c32753c52873d5ef2
diff --git a/types.rst b/types.rst
index fbbc715..ded24c7 100644
--- a/types.rst
+++ b/types.rst
@@ -136,6 +136,8 @@
 +----------------+-----------------------------------------------------------+
 | 19             | Reserved, formerly *Any*                                  |
 +----------------+-----------------------------------------------------------+
+| 31             | Reserved, formerly *Delegation*                           |
++----------------+-----------------------------------------------------------+
 | [80, 100]      | Reserved for assignments related to local-link data       |
 |                | processing, e.g., NDNLP_ (1-byte encoding)                |
 +----------------+-----------------------------------------------------------+