CH6/7/8: update link anchors for consistency

chapters/signatures.adoc

@@ -1,4 +1,4 @@
-[[signatures]]
+[[c_signatures]]
 === Signatures
 
 FIXME

chapters/transactions.adoc

@@ -1,4 +1,4 @@
-[[transactions]]
+[[c_transactions]]
 == Transactions
 
 The way we typically transfer physical cash has little resemblance to
@@ -712,7 +712,7 @@ protected by the following script:
 ----
 
 Obviously, allowing your bitcoins to be spent by anyone who can solve a
-simple equation wouldn't be secure.  As we'll see in <<signatures>>, an
+simple equation wouldn't be secure.  As we'll see in <<c_signatures>>, an
 unforgable digital signature scheme uses an equation that can only be
 solved by someone in possession of certain data which they're able to
 keep secret.  They're able to reference that secret data using a public