build(using-contentful): add missing sharp dependency (#30546)

Author: axe312ger

examples/using-contentful/package.json

@@ -10,6 +10,7 @@
     "gatsby-plugin-google-analytics": "next",
     "gatsby-plugin-image": "next",
     "gatsby-plugin-offline": "next",
+    "gatsby-plugin-sharp": "next",
     "gatsby-plugin-typography": "next",
     "gatsby-source-contentful": "next",
     "gatsby-transformer-remark": "next",