Fix typo in documentation of CanvasElement.toDataUrl.

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 239 matching lines @@
           "   *",
           "   * If the type requested is not 'image/png', and the returned value is",
           "   * 'data:image/png', then the requested type is not supported.",
           "   *",
           "   * Example usage:",
           "   *",
           "   *     CanvasElement canvas = new CanvasElement();",
           "   *     var ctx = canvas.context2D",
           "   *     ..fillStyle = \"rgb(200,0,0)\"",
           "   *     ..fillRect(10, 10, 55, 50);",
-          "   *     var dataUrl = canvas.toDataURL(\"image/jpeg\", 0.95);",
+          "   *     var dataUrl = canvas.toDataUrl(\"image/jpeg\", 0.95);",
           "   *     // The Data Uri would look similar to",
           "   *     // 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA",
           "   *     // AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO",
           "   *     // 9TXL0Y4OHwAAAABJRU5ErkJggg=='",
           "   *     //Create a new image element from the data URI.",
           "   *     var img = new ImageElement();",
           "   *     img.src = dataUrl;",
           "   *     document.body.children.add(img);",
           "   *",
           "   * See also:",
@@ skipping 344 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }