assert.3: Update manual page based on mandoc -Tlint

Execute the mandoc -Tlint option on assert(3). This results in two areas
of updates:

Remove invalid Rs block
Remove invalid Re block

Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1132

(cherry picked from commit 3f72f36cf2f92e00a248f729d50803232ff4cef8)