Documentation: change lineColor to strokeColor for the first 3 example charts

Change lineColor to strokeColor in legendTemplate for the example code for the Line, Bar, and Radar chart types in the docs. Otherwise a copy-and-paste of the example code will lead to a blank <span style="background-color:"></span> situation where the spans in the legend have no background-color to which to refer.
Actually in the `Chart.Bar.js` file, the default is `"<ul class=\"<%=name.toLowerCase()%>-legend\"><% for (var i=0; i<datasets.length; i++){%><li><span style=\"background-color:<%=datasets[i].fillColor%>\"></span><%if(datasets[i].label){%><%=datasets[i].label%><%}%></li><%}%></ul>"`.