crypto: MAINTAINERS - fix file path for SHA multibuffer code

"arch/x86/crypto/sha*-mb" needs a trailing slash, since it refers to
directories.  Otherwise get_maintainer.pl doesn't find the entry.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>