docs: append setting to make gRPC work (#5198)

* docs: append setting to make gRPC work

* Update _traefik.mdx

* Update _traefik.mdx

---------

Co-authored-by: Elio Bischof <eliobischof@gmail.com>

docs/docs/self-hosting/deploy/loadbalancing-example/example-traefik.yaml

@@ -23,6 +23,8 @@ http:
         isDevelopment: false
         allowedHosts:
         - 'my.domain'
+        customRequestHeaders:
+          :authority: 'my.domain'
     redirect-to-https:
       redirectScheme:
         scheme: https

docs/docs/self-hosting/manage/reverseproxy/_traefik.mdx

@@ -20,6 +20,8 @@ http:
         isDevelopment: false
         allowedHosts:
         - 'localhost'
+        customRequestHeaders:
+          :authority: 'localhost'
     redirect-to-https:
       redirectScheme:
         scheme: https
@@ -76,6 +78,8 @@ http:
         isDevelopment: false
         allowedHosts:
         - 'localhost'
+        customRequestHeaders:
+          :authority: 'localhost'
     redirect-to-https:
       redirectScheme:
         scheme: https
@@ -127,6 +131,8 @@ http:
         isDevelopment: false
         allowedHosts:
         - 'localhost'
+        customRequestHeaders:
+          :authority: 'localhost'
   routers:
     router0:
       entryPoints:
@@ -141,4 +147,4 @@ http:
         servers:
         - url: h2c://localhost:8080
         passHostHeader: true
-```
+```