INSTALL: Fix setting content-type on well-known

INSTALL.md

@@ -487,7 +487,7 @@ In nginx this would be something like:
 ```
 location /.well-known/matrix/client {
     return 200 '{"m.homeserver": {"base_url": "https://<matrix.example.com>"}}';
-    add_header Content-Type application/json;
+    default_type application/json;
     add_header Access-Control-Allow-Origin *;
 }
 ```

changelog.d/8793.doc

@@ -0,0 +1 @@
+Fix the example on how to set the `Content-Type` header in nginx for the Client Well-Known URI.