blue-green-upgrade.md: Use canonical Docusaurus link

I.e. drop the trailing forward slash.

Author: dumbbell

docs/blue-green-upgrade.md

@@ -23,7 +23,7 @@ limitations under the License.
 
 ## Overview {#overview}
 
-Blue-green deployment is an [upgrade strategy](./upgrade/) that is based on the idea of setting up
+Blue-green deployment is an [upgrade strategy](./upgrade) that is based on the idea of setting up
 a second RabbitMQ cluster (the "green" one) next to the current production
 cluster (the "blue" one). Applications are then switched to the "green"
 cluster. When that migration is done, the "blue" cluster is decommissioned (shut down).