fix(docs): correct wording in indexCounter comment

packages/material-ui-styles/src/makeStyles/indexCounter.js

@@ -1,6 +1,6 @@
 /* eslint-disable import/prefer-default-export */
 // Global index counter to preserve source order.
-// We create the style sheet during at the creation of the component,
+// We create the style sheet during the creation of the component,
 // children are handled after the parents, so the order of style elements would be parent->child.
 // It is a problem though when a parent passes a className
 // which needs to override any child's styles.