fix #1912 docs error

Author: rezrah

docs/content/Header.mdx

@@ -16,7 +16,7 @@ All items directly under the Header component should be a `Header.Item` componen
 <Header>
   <Header.Item>
     <Header.Link href="#" fontSize={2}>
-      <StyledOcticon icon={MarkGithubIcon} size={32} mr={2} />
+      <StyledOcticon icon={MarkGithubIcon} size={32} sx={{mr: 2}} />
       <span>GitHub</span>
     </Header.Link>
   </Header.Item>