fix typo

Author: etpinard

CONTRIBUTING.md

@@ -17,7 +17,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 | `type: bug` | bug report confirmed by a plotly team member |
 | `type: feature` | planned feature additions |
 | `type: performance` | performance related tasks |
-| `type: maintenace` | source code cleanup resulting in no enhancement for users |
+| `type: maintenance` | source code cleanup resulting in no enhancement for users |
 | `type: documentation` | API doc or attribute description improvements |
 | `type: community` | issue left open for community input and pull requests |
 | `type: duplicate` | *self-explanatory* |