doc: ARM: Use the right function name

Trivial fix.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

doc/README.arm-caches

@@ -47,7 +47,7 @@ Cleanup Before Linux:
 - cleanup_before_linux() should flush the D-cache, invalidate I-cache, and
   disable MMU and caches.
 - The following sequence is advisable while disabling d-cache:
-  1. disable_dcache() - flushes and disables d-cache
+  1. dcache_disable() - flushes and disables d-cache
   2. invalidate_dcache_all() - invalid any entry that came to the cache
 	in the short period after the cache was flushed but before the
 	cache got disabled.