Define with let to avoid "assignment to constant" errors

Thanks for this example. Defining `label` with `const` rather than `let` results in `Uncaught TypeError: Assignment to constant variable.`

docs/configuration/tooltip.md

@@ -157,7 +157,7 @@ const chart = new Chart(ctx, {
             tooltip: {
                 callbacks: {
                     label: function(context) {
-                        const label = context.dataset.label || '';
+                        let label = context.dataset.label || '';
 
                         if (label) {
                             label += ': ';