interest: Improve BNF of Exclude selector

The previous definition of the Exclude selector BNF allows invalid
filters of the form:

    Any NameComponent Any
    NameComponent Any NameComponent Any

Change-Id: I7e40e3b1cf79b30fbee5c5bccc5b1ec52e82352a
diff --git a/interest.rst b/interest.rst
index 55fa597..a6d7460 100644
--- a/interest.rst
+++ b/interest.rst
@@ -81,7 +81,10 @@
 
 ::
 
-    Exclude ::= EXCLUDE-TYPE TLV-LENGTH Any? (NameComponent (Any)?)+
+    Exclude ::= EXCLUDE-TYPE TLV-LENGTH AnyLeading? (NameComponent | AnyBetween)* AnyTrailing?
+    AnyLeading ::= Any NameComponent
+    AnyBetween ::= NameComponent Any NameComponent
+    AnyTrailing ::= NameComponent Any
     Any ::= ANY-TYPE TLV-LENGTH(=0)
 
 The ``Exclude`` selectors allow requesters to specify list and/or ranges of name components that MUST NOT appear as a continuation of the Name prefix in the responding Data packet to the Interest.