fix: Update no-duplicate-heading rule for markdownlint

The rule was adjusted to no longer use `no-duplicate-header`, and soon
will no longer support the `allow_different_nesting` parameter. Fixed
the rule to use the newest alias.

Author: joshuacurtiss

.markdownlint.json

@@ -11,8 +11,8 @@
       "code_block_line_length": 160,
       "tables": false
    },
-   "no-duplicate-header": {
-      "allow_different_nesting": true
+   "no-duplicate-heading": {
+      "siblings_only": true
    },
    "hr-style": { "style": "---" },
    "code-block-style": { "style": "fenced" },