Fix video width warnings, using integers instead of percentage

Author: stevepiercy

docs/contributing/myst-reference.md

@@ -261,7 +261,7 @@ Example MyST syntax is shown below.
 
 ````md
 ```{video} /_static/user-manual/blocks/block-copy-cut.mp4
-    :width: 100%
+    :width: 100
 ```
 ````
 
@@ -270,7 +270,7 @@ Don't use file-relative paths.
 The above MyST markup renders as shown below.
 
 ```{video} /_static/user-manual/blocks/block-copy-cut.mp4
-    :width: 100%
+    :width: 100
 ```