Fix title render issue of Digest Authentication document

Closes gh-11272

docs/modules/ROOT/pages/servlet/authentication/passwords/digest.adoc

@@ -1,4 +1,4 @@
-**[[**servlet-authentication-digest]]
+[[servlet-authentication-digest]]
 = Digest Authentication
 
 This section provides details on how Spring Security provides support for https://tools.ietf.org/html/rfc2617[Digest Authentication], which is provided `DigestAuthenticationFilter`.