[docs] Use new email for sponsoring

We want to test Front in anticipation of using it for Custom Success.
Based on the experience, we could either kill Front and move to Freshdesk or stay on it.

docs/data/material/discover-more/backers/backers.md

@@ -134,7 +134,7 @@ Your organization logo will be prominently featured:
 - the [README.md](https://github.com/mui/material-ui#sponsors) (80k+ unique visitors/month)
 - the [Diamond sponsor list](#diamond-sponsors)
 
-Please contact us at sales@mui.com before subscribing to this tier to get preliminary approval.
+Please contact us at sponsoring@mui.com before subscribing to this tier to get preliminary approval.
 
 ### Gold