fix internal reference. fixes #205.

index.bs

@@ -707,8 +707,8 @@ This is a dictionary containing zero or more extensions as defined in [[#extensi
 that can be passed to the <a>getAssertion()</a> method and triggers some additional processing by the client platform and/or the
 authenticator.
 
-If the caller wants to pass extensions to the platform, it SHOULD do so by adding one entry per extension to this dictionary
-with the extension identifier as the key, and the extension's value as the value (see [[#signature-format]] for details).
+If the caller wishes to pass extensions to the platform, it MUST do so by adding one entry per extension to this dictionary
+with the extension identifier as the key, and the extension's value as the value (see [[#extensions]] for details).
 
 
 ## Credential Attestation Structure (interface <dfn interface>WebAuthnAttestation</dfn>) ## {#iface-attestation}