fix: remove Missing

Author: Sudrut

src/content/docs/cpp/language/basic_concepts/main_function.mdx

@@ -8,7 +8,6 @@ import Behavior from "@components/Behavior.astro";
 import { Decl, DeclDoc } from "@components/decl-doc";
 import { DR, DRList } from "@components/defect-report";
 import { Desc, DescList, DocLink } from '@components/index';
-import Missing from "@components/Missing.astro";
 import { ParamDoc, ParamDocList } from "@components/param-doc";
 import { Revision } from "@components/revision";
 import WG21PaperLink from "@components/WG21PaperLink.astro";