Fix typo in server variables schema for OAS 3.1.0

Fixes #197

Author: samdbmg

openapi_spec_validator/resources/schemas/v3.1/schema.json

@@ -189,7 +189,7 @@
         "default": {
           "type": "string"
         },
-        "descriptions": {
+        "description": {
           "type": "string"
         }
       },