self is immutable in the Plugin trait's methods (#3555)

Fix #3539

.changesets/docs_geal_plugin_doc_fix.md

@@ -0,0 +1,4 @@
+### self is immutable in the Plugin trait's methods ([Issue #3539](https://github.com/apollographql/router/issues/3539))
+
+
+By [@Geal](https://github.com/Geal) in https://github.com/apollographql/router/pull/3555

docs/source/customizations/native.mdx

@@ -96,21 +96,21 @@ impl Plugin for HelloWorld {
     // Only define the hooks you need to modify. Each default hook
     // implementation returns its associated service with no changes.
     fn router_service(
-        &mut self,
+        &self,
         service: router::BoxService,
     ) -> router::BoxService {
         service
     }
 
     fn supergraph_service(
-        &mut self,
+        &self,
         service: router::BoxService,
     ) -> router::BoxService {
         service
     }
 
     fn execution_service(
-        &mut self,
+        &self,
         service: execution::BoxService,
     ) -> execution::BoxService {
         service
@@ -120,7 +120,7 @@ impl Plugin for HelloWorld {
     // being invoked. That's because this service might invoke *multiple*
     // subgraphs for a single request, and this is called once for each.
     fn subgraph_service(
-        &mut self,
+        &self,
         name: &str,
         service: subgraph::BoxService,
     ) -> subgraph::BoxService {
@@ -141,7 +141,7 @@ use tower::ServiceBuilderExt;
 use apollo_router::ServiceBuilderExt as ApolloServiceBuilderExt;
 
 fn supergraph_service(
-    &mut self,
+    &self,
     service: router::BoxService,
 ) -> router::BoxService {
     // Always use service builder to compose your plugins.