doc: MiB is mebibytes/1024*1024

The doc is using 'mibibytes', and 1024*1024*1024, this commit fixes both
issues, and adds a link to the wikipedia article.

This fixes #176

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>

doc/usage.md

@@ -30,7 +30,7 @@ Number of virtual CPUs (vCPU) available in the VM. It defaults to 1 vCPU.
 
 - `--memory`
 
-Amount of memory available in the virtual machine. The value is in MiB (mibibytes, 1024 * 1024 * 1024 bytes), and the default is 512 MiB.
+Amount of memory available in the virtual machine. The value is in MiB ([mebibytes](https://simple.wikipedia.org/wiki/Mebibyte), 1024 * 1024 bytes), and the default is 512 MiB.
 
 ### Time Synchronization Configuration