chore: remove/update pinned gatsby-remark-prismjs plugin (#1253)

Author: DSchau

package.json

@@ -34,7 +34,7 @@
     "gatsby-remark-copy-linked-files": "^2.0.0",
     "gatsby-remark-embed-snippet": "^3.0.0",
     "gatsby-remark-images": "^2.0.0",
-    "gatsby-remark-prismjs": "3.0.0-beta.6",
+    "gatsby-remark-prismjs": "^3.0.2",
     "gatsby-remark-responsive-iframe": "^2.0.0",
     "gatsby-remark-smartypants": "^2.0.0",
     "gatsby-source-filesystem": "^2.0.0",