Fix reference to Null Object pattern in the manual

docs/manual/src/docs/asciidoc/index.adoc

@@ -2736,7 +2736,7 @@ The default implementation is `HttpSessionSecurityContextRepository`, which stor
 </bean>
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-Alternatively you could provide an instance of `NullSecurityContextRepository`, a "http://en.wikipedia.org/wiki/Null_Object_pattern[null object]" implementation, which will prevent the security context from being stored, even if a session has already been created during the request.
+Alternatively you could provide an instance of `NullSecurityContextRepository`, a http://en.wikipedia.org/wiki/Null_Object_pattern[null object] implementation, which will prevent the security context from being stored, even if a session has already been created during the request.
 
 
 [[form-login-filter]]