shrink ramdisk a teeny bit; 8MB installs now work reliably

src/etc/etc.i386/disktab

@@ -119,6 +119,6 @@ fuji513|Fujitsu M22XXXX: \
 
 rdroot|ramdiskroot|RAM-disk root FS image:\
 	:ty=ramdisk:se#512:nt#2:ns#128:nc#16:\
-	:pa#3560:oa#0:ta=4.2BSD:ba#4096:fa#512:\
+	:pa#3510:oa#0:ta=4.2BSD:ba#4096:fa#512:\
 	:pb#0:ob#0:tb=swap:\
-	:pc#3560:oc#0:
+	:pc#3510:oc#0: