Update spec_url of @charset from css2 to css3 (#25574)

Author: skyclouds2001

css/at-rules/charset.json

@@ -5,7 +5,7 @@
         "__compat": {
           "description": "`@charset`",
           "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@charset",
-          "spec_url": "https://drafts.csswg.org/css2/#charset①",
+          "spec_url": "https://drafts.csswg.org/css-syntax/#at-ruledef-charset",
           "tags": [
             "web-features:charset"
           ],