Update shapes.md

Author: LiamConnors

doc/python/shapes.md

@@ -917,7 +917,7 @@ fig.show()
 
 *New in 5.15*
 
-Use `texttemplate` to add text with variables to shapes. `texttemplate` uses d3 number and date formatting and supports raw variables, which use the raw data from the shape definition, and some calculated variables. 
+Use `texttemplate` to add text with variables to shapes. `texttemplate` uses d3 number and date formatting and supports raw variables, which use the raw data from the shape definition, and some calculated variables. Add a variable with "%{variable}".
 
 This example adds the raw variables `x0` and `y0` to a rectangle and shows the calculated variables `height`, `slope`, and `width`on three other shapes.