Fix a typo in an address_set.c comment.

/** How many 64-bit siphash values to extract per address */

#define N_HASHES 2

/** How many bloom-filter bits we set per address. This is twice the N_HASHES

 * value, since we split the siphash outcome two 32-bit values. */

 * value, since we split the siphash output into two 32-bit values. */

#define N_BITS_PER_ITEM (N_HASHES * 2)

/* XXXX This code is largely duplicated with digestset_t.  We should merge