Force vomit to look for CPE from the README file

Per suggestion from the vomit team, in http://b/295311007#comment25,
add the CPEPrefix causes vomit to look for CPE information from the
README file (Version:) rather than from the vomit database which
had an outdated hardcoded CPE for this jinja2 package.

Bug: 472035343
Change-Id: Ie72567ea664db8bcfc6a0a4a849463eee6b8708c
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/jinja2/+/1464593
Reviewed-by: Eric Stone <ecstone@google.com>