Argon2: Fix docs about minimum value of m_cost

[sorairolake]

According to RFC 9106:

Memory size m MUST be an integer number of kibibytes from 8*p to
2^(32)-1. The actual number of blocks is m', which is m rounded
down to the nearest multiple of 4*p.

8 is the minimum value of m_cost when p_cost is 1, and the minimum value of m_cost changes depending on the value of p_cost, so I fix the explanation about the minimum value of m_cost.

See also: #458 (comment)