Fix minimum size description of challenge

The server-provided challenge is almost always smaller than 32 bytes,
so we cannot enforce that as a minimum. I fixed up the CDDL a while
back, but missed one mention of the 32 byte minimum in the description.

Test: n/a (it's a comment)
Bug: 272392463
Change-Id: Ia5994e2b7cf107ab131c6b028bee7881d0e657ac