doc: added renamed chart fields to the 12.0 migration guide

In primefaces#6812 org.primefaces.model.charts.line.LineChartDataSet attributes
lineTension and lineStepped were renamed to tension and stepped
respectively.
This requires client code to be updated when migrating to primefaces 12

docs/migrationguide/12_0_0.md

@@ -23,6 +23,8 @@
   * `scales.axes.gridLines` was renamed to `scales.axes.grid`
   * `scales.axes.gridLines.offsetGridLines` was renamed to `scales.axes.grid.offset`
   * `scales.axes.gridLines.tickMarkLength` was renamed to `scales.axes.grid.tickLength`
+  * `charts.line.lineTension` was renamed to `charts.line.tension`
+  * `charts.line.steppedLine` was renamed to `charts.line.stepped`
 
 ## GMap
   * Marker `icon` type is now `Object` instead of `String` (to support `Symbol`).