Correct the example Python code for the border style (#1956)

To the best of my knowledge, and in testing myself, border and border_* are
properties of a widget's `styles` property, they're not a property of the
widget itself.

docs/styles/border.md

@@ -134,10 +134,10 @@ border-left: outer red;
 
 ```python
 # Set a heavy white border
-widget.border = ("heavy", "white")
+widget.styles.border = ("heavy", "white")
 
 # Set a red border on the left
-widget.border_left = ("outer", "red")
+widget.styles.border_left = ("outer", "red")
 ```
 
 ## See also