Fixes broken GatewayFilter link in documentation

Fixes gh-1880
Fixes gh-1881

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -26,7 +26,7 @@ It does not work in a traditional Servlet Container or when built as a WAR.
 It is defined by an ID, a destination URI, a collection of predicates, and a collection of filters. A route is matched if the aggregate predicate is true.
 * *Predicate*: This is a https://docs.oracle.com/javase/8/docs/api/java/util/function/Predicate.html[Java 8 Function Predicate]. The input type is a https://docs.spring.io/spring/docs/5.0.x/javadoc-api/org/springframework/web/server/ServerWebExchange.html[Spring Framework `ServerWebExchange`].
 This lets you match on anything from the HTTP request, such as headers or parameters.
-* *Filter*: These are instances of https://docs.spring.io/spring/docs/5.0.x/javadoc-api/org/springframework/web/server/GatewayFilter.html[Spring Framework `GatewayFilter`] that have been constructed with a specific factory.
+* *Filter*: These are instances of {github-code}/spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/GatewayFilter.java[`GatewayFilter`] that have been constructed with a specific factory.
 Here, you can modify requests and responses before or after sending the downstream request.
 
 [[gateway-how-it-works]]