Fix dependabot configuration typo (#631)

I am not sure why this change is not validated on pull request :(

Follows #630

Author: zanieb

.github/dependabot.yml

@@ -1,6 +1,6 @@
 version: 2
 updates:
-- package-ecosystem: "pip
+- package-ecosystem: "pip"
   directory: "/"
   schedule:
     interval: "weekly"