Fix typo and ASN.1 tag number range comment in bytestring.h.

CBS_get_asn1() and CBS_get_any_asn1_element() only support the single
byte ASN.1 identifier octets (aka short form tags).  Tag number 31 is
the start of the multi-byte long form per X.690 section 8.1.2.4.

Change-Id: I892600f4946e880a4ff03d219181116ef04f5372
Reviewed-on: https://boringssl-review.googlesource.com/3241
Reviewed-by: Adam Langley <agl@google.com>