doc: fix README.standalone

Since commit fe1378a961e5 (ARM: use r9 for gd), r9 register
is used on ARM to hold global_data.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>

doc/README.standalone

@@ -21,7 +21,7 @@ Design Notes on Exporting U-Boot Functions to Standalone Applications:
 2. The pointer to the jump table is passed to the application in a
    machine-dependent way. PowerPC, ARM, MIPS, Blackfin and Nios II
    architectures use a dedicated register to hold the pointer to the
-   'global_data' structure: r2 on PowerPC, r8 on ARM, k0 on MIPS,
+   'global_data' structure: r2 on PowerPC, r9 on ARM, k0 on MIPS,
    P3 on Blackfin and gp on Nios II. The x86 architecture does not
    use such a register; instead, the pointer to the 'global_data'
    structure is passed as 'argv[-1]' pointer.