docs(docker-compose): Fix obsolete repo-cache volume declaration (#741)

compose/ferretdb.compose.yaml

@@ -90,6 +90,4 @@ volumes:
   # Postgres
   postgres-data:
   # FerretDB
-  ferretdb-state:
+  ferretdb-state:
-  # Core
-  repo-cache:

compose/mongo.compose.yaml

@@ -72,6 +72,4 @@ services:
 volumes:
   # Mongo
   mongo-data:
-  mongo-config:
+  mongo-config:
-  # Core
-  repo-cache: