Repoint .istambul.yml to the right location (#5187)

Never did this when I renamed the folder. `npm run cover` currently
doesn't generate a proper coverage report, this fixes it

superset/assets/.istanbul.yml

@@ -1,6 +1,6 @@
 verbose: true
 instrumentation:
-    root: './javascripts'
+    root: './src'
     extensions: ['.js', '.jsx']
     excludes: [
         'dist/**'