doc: README.ae350: Sync for ax25-ae350 rename

Rename ax25-ae350 as ae350_rv[32|64] for 32 or 64 bit.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/README.ae350 b/doc/README.ae350
index fe75b80..189a6b7 100644
--- a/doc/README.ae350
+++ b/doc/README.ae350
@@ -25,7 +25,7 @@
 
 build:
 1. Prepare the toolchains and make sure the $PATH to toolchains is correct.
-2. Use `make ax25-ae350_defconfig` in u-boot root to build the image.
+2. Use `make ae350_rv[32|64]_defconfig` in u-boot root to build the image for 32 or 64 bit.
 
 Verification
 ====================