Fix typo in signature definition

Change-Id: I7006d696f1e7ed2d8492190388e50166a3aaaf20
Refs: #2897
diff --git a/signature.rst b/signature.rst
index 47cf123..16a02e2 100644
--- a/signature.rst
+++ b/signature.rst
@@ -15,7 +15,7 @@
 ::
 
     Signature ::= SignatureInfo
-                  SignatureBits
+                  SignatureValue
 
     SignatureInfo ::= SIGNATURE-INFO-TYPE TLV-LENGTH
                         SignatureType