Update custom-buttons.md

Set the 'updatemenu method' link directly to the method instead of the page header.

Author: SylwiaOliwia2

doc/python/custom-buttons.md

@@ -34,7 +34,7 @@ jupyter:
 ---
 
 #### Methods
-The [updatemenu method](https://plot.ly/python/reference/#layout-updatemenus-buttons-method) determines which [plotly.js function](https://plot.ly/javascript/plotlyjs-function-reference/) will be used to modify the chart. There are 4 possible methods:
+The [updatemenu method](https://plot.ly/python/reference/#layout-updatemenus-items-updatemenu-buttons-items-button-method) determines which [plotly.js function](https://plot.ly/javascript/plotlyjs-function-reference/) will be used to modify the chart. There are 4 possible methods:
 - `"restyle"`: modify data or data attributes
 - `"relayout"`: modify layout attributes
 - `"update"`: modify data **and** layout attributes