recipes-rt: update README to match newer override syntax

The override syntax uses colon (':') instead of underscore ('_') since
Honister (3.4), so let's update the README to use that new syntax.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From OE-Core rev: 97f0a73c0a9db5506310328cd29f165eab3c509d)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-rt/README

@@ -10,4 +10,4 @@ If you are creating a new BSP which should use linux-yocto-rt by default,
 use the line above in the $MACHINE.conf in your BSP layer, and specify the
 following in a linux-yocto-rt bbappend recipe:
 
-COMPATIBLE_MACHINE_$MACHINE = $MACHINE
+COMPATIBLE_MACHINE:$MACHINE = $MACHINE