Fix doc tests

Signed-off-by: Christopher N. Hesse <raymanfx@gmail.com>

eye-hal/src/lib.rs

@@ -44,7 +44,7 @@
 //!
 //! // First, we need a capture device to read images from. For this example, let's just choose
 //! // whatever device is first in the list.
-//! let dev = ctx.open_device(&devices[0]).expect("Failed to open video device");
+//! let dev = ctx.open_device(&devices[0].uri).expect("Failed to open video device");
 //!
 //! // Query for available streams and just choose the first one.
 //! let streams = dev.streams().expect("Failed to query streams");

eye-hal/src/platform/mod.rs

@@ -52,6 +52,14 @@ impl<'a> Context<'a> {
     }
 }
 
+impl<'a> Default for Context<'a> {
+    fn default() -> Self {
+        Self::all()
+            .next()
+            .expect("no contexts available for this platform")
+    }
+}
+
 impl<'a> ContextTrait<'a> for Context<'a> {
     type Device = Device<'a>;