fix: improve config schema

schema/html-w3c-validatorrc-schema.json

@@ -7,6 +7,7 @@
     "$schema": {
       "description": "JSON Schema URI",
       "type": "string",
+      "format": "uri",
       "default": "https://raw.githubusercontent.com/theoludwig/html-w3c-validator/main/schema/html-w3c-validatorrc-schema.json"
     },
     "urls": {
@@ -17,6 +18,7 @@
       "items": {
         "description": "URL to validate.",
         "type": "string",
+        "format": "uri",
         "minLength": 1
       }
     },
@@ -28,11 +30,12 @@
       "items": {
         "description": "File to validate.",
         "type": "string",
+        "format": "uri-reference",
         "minLength": 1
       }
     },
     "severities": {
-      "description": "Specify the severities of the validator messages to report.",
+      "description": "Specify the severities to report.",
       "type": "array",
       "minItems": 1,
       "default": ["warning", "error"],