Correct the release date for 2.11BSD

2.11BSD was announced on March 14, 1991 in comp.bugs.2bsd by
Steven M. Schultz. The document has a 'revised January 1991'
date at the top.

Patch/1 in the official repo is dated March 31, 1991, and an identical copy of
it was posted to comp.bugs.2bsd on May 5, 1991. Patch 2 in 22 parts was likewise
posted May 18, 1991. This makes the Feb 1992 date too late. It's possible it's a
typo for Feb 1991 since that lines up with the announcement being 2 weeks
later. Without an extant copy of the 2.11 tape, however, it's hard to say for
sure. Go with the date we have the most independent, direct evidence for, which
is the announcement date.