doc: verified-boot: fix crypto algorithm examples

As you see in crypto_algos in common/image-sig.c, the algorithm
should be either "rsa2048" or "rsa4096".  "rs2048" is a typo.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>