Move Mutual TLS example from 3.1.0 to 3.1.1 to prepare for patch release

Author: lornajane

versions/3.1.1.md

@@ -3250,6 +3250,20 @@ scheme: bearer
 bearerFormat: JWT
 ```
 
+###### MutualTLS Sample
+
+```json
+{
+  "type": "mutualTLS",
+  "description": "Cert must be signed by example.com CA"
+}
+```
+
+```yaml
+type: mutualTLS
+description: Cert must be signed by example.com CA
+```
+
 ###### Implicit OAuth2 Sample
 
 ```json