Fix youtube video resolution

- With the new theme, setting fixed 720p resolution makes it overflow
  horizontally. This looks quite odd.
- The guide to add video was already updated to use 100% width instead
  of fixed size, so just update some older uses.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

boards/beagleplay/02-quick-start.rst

@@ -27,8 +27,7 @@ Attaching antennas
 You can watch this video to see how to attach the attennas.
 
 .. youtube:: 8zeIVd-JRc0
-    :width: 1280
-    :height: 720
+    :width: 100%
     :align: center
 
 Tethering to PC

boards/beaglev/ahead/02-quick-start.rst

@@ -24,8 +24,7 @@ Unboxing
 *********
 
 .. youtube:: SVC9peUUzE0
-   :width: 1280
-   :height: 720
+   :width: 100%
    :align: center
 
 Antenna guide