gitignore: Fix RAM file name

From:
    commit 8e0389b973
    Author: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Date:   Wed Oct 23 12:08:55 2019 +1100
    ram: Rework main RAM interface

We need to change the name.

Signed-off-by: Michael Neuling <mikey@neuling.org>

.gitignore

@@ -3,4 +3,4 @@
 *.cf
 *.s
 *_tb
-simple_ram_behavioural.bin
+main_ram.bin