Change url-ignore option to ignore-urls in workflow

For 4+ we need to change url-ignore to ignore-urls.

Author: ShawnMcKee

.github/workflows/validate-mkdocs.yml

@@ -59,4 +59,4 @@ jobs:
           htmlproofer ./site \
             --assume-extension \
             --check-external-hash \
-            --url-ignore "$URL_IGNORE"
+            --ignore-urls "$URL_IGNORE"