Bump node-fetch from 2.6.0 to 2.6.7

Bumps [node-fetch](https://github.com/node-fetch/node-fetch) from 2.6.0 to 2.6.7.
- [Release notes](https://github.com/node-fetch/node-fetch/releases)
- [Commits](node-fetch/node-fetch@v2.6.0...v2.6.7)

---
updated-dependencies:
- dependency-name: node-fetch
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -242,7 +242,7 @@
 		"mathjax": "^3.0.1",
 		"mathjax-full": "^3.0.1",
 		"mobx": "^5.15.4",
-		"node-fetch": "^2.6.0",
+		"node-fetch": "^2.6.7",
 		"string": "git+ssh://git@github.com/hackmdio/string.js.git#a68176b3d"
 	}
 }