chore(docs): Add props to MyParagraph (#27124)

Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>

Author: kai687

packages/gatsby-plugin-mdx/README.md

@@ -475,7 +475,9 @@ all of the MDX content.
 import { MDXProvider } from "@mdx-js/react"
 
 const MyH1 = props => <h1 style={{ color: "tomato" }} {...props} />
-const MyParagraph = props => <p style={{ fontSize: "18px", lineHeight: 1.6 }} />
+const MyParagraph = props => (
+  <p style={{ fontSize: "18px", lineHeight: 1.6 }} {...props} />
+)
 
 const components = {
   h1: MyH1,