Add and fix examples comments

Author: maan2003

druid/examples/hello.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 //! This is a very small example of how to setup a druid application.
-//! It does the almost bare minimum while still being usefull.
+//! It does the almost bare minimum while still being useful.
 
 use druid::widget::prelude::*;
 use druid::widget::{Flex, Label, TextBox};

druid/examples/lens.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//! This example shows basic usage of Lens
+
 use druid::widget::Slider;
 use druid::widget::{CrossAxisAlignment, Flex, Label, TextBox};
 use druid::{AppLauncher, Data, Env, Lens, LocalizedString, Widget, WidgetExt, WindowDesc};

druid/examples/open_save.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//! Usage of file open and saving.
+
 use druid::widget::{Align, Button, Flex, TextBox};
 use druid::{
     commands, AppDelegate, AppLauncher, Command, DelegateCtx, Env, FileDialogOptions, FileSpec,