Update RAND_DRBG.pod

Fixed a minor typo while reading the documentation.
I agree that this contribution is trivial can be freely used.

CLA: trivial

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/7221)