Correctly document minimumTlsVersion - which is create-only mutable (#12795)

* Correctly document minimumTlsVersion - which is really only configurable at create time.

* Style fix for empty object

Author: TimLovellSmith

specification/redisenterprise/resource-manager/Microsoft.Cache/preview/2020-10-01-preview/examples/RedisEnterpriseUpdate.json

@@ -9,9 +9,7 @@
         "name": "EnterpriseFlash_F300",
         "capacity": 9
       },
-      "properties": {
-        "minimumTlsVersion": "1.2"
-      },
+      "properties": {},
       "tags": {
         "tag1": "value1"
       }

specification/redisenterprise/resource-manager/Microsoft.Cache/preview/2020-10-01-preview/redisenterprise.json

@@ -1259,6 +1259,10 @@
       "properties": {
         "minimumTlsVersion": {
           "type": "string",
+          "x-ms-mutability": [
+            "read",
+            "create"
+          ],
           "description": "The minimum TLS version for the cluster to support, e.g. '1.2'"
         },
         "hostName": {