ReseedingRng: fix doc comment on reseeding behavior after UNIX fork (#1317)

It reads "For ChaCha and Hc128 this is a maximum of fifteen `u32` values before
reseeding" while rand_chacha v0.2.0 or higher actually consumes 63 `u32` values
before reseeding.

Author: LiosK

src/rngs/adapter/reseeding.rs

@@ -24,8 +24,8 @@ use rand_core::{CryptoRng, Error, RngCore, SeedableRng};
 /// - After `clone()`, the clone will be reseeded on first use.
 /// - When a process is forked on UNIX, the RNGs in both the parent and child
 ///   processes will be reseeded just before the next call to
-///   [`BlockRngCore::generate`], i.e. "soon". For ChaCha and Hc128 this is a
-///   maximum of fifteen `u32` values before reseeding.
+///   [`BlockRngCore::generate`], i.e. "soon". For ChaCha and Hc128, this is a
+///   maximum of 63 and 15, respectively, `u32` values before reseeding.
 /// - After the PRNG has generated a configurable number of random bytes.
 ///
 /// # When should reseeding after a fixed number of generated bytes be used?