update gatsby-plugin-mdx to version 5.15.0 in package.json and package-lock.json

Signed-off-by: Lee Calcote <lee.calcote@layer5.io>

Author: leecalcote

package-lock.json

@@ -64,7 +64,7 @@
     "gatsby-plugin-image": "^3.11.0",
     "gatsby-plugin-loadable-components-ssr": "^4.3.2",
     "gatsby-plugin-manifest": "^5.15.0",
-    "gatsby-plugin-mdx": "^5.0.0",
+    "gatsby-plugin-mdx": "^5.15.0",
     "gatsby-plugin-meta-redirect": "github:layer5labs/gatsby-plugin-meta-redirect",
     "gatsby-plugin-netlify": "^5.1.1",
     "gatsby-plugin-preload-fonts": "^4.11.0",