Fixed typo in release notes for 1.55

git-svn-id: http://svn.boost.org/svn/boost/branches/release@86181 b8fc166d-592f-0410-95f2-cb63ce0dd405
Fixed typo in release notes for 1.55 (again)

git-svn-id: http://svn.boost.org/svn/boost/branches/release@86182 b8fc166d-592f-0410-95f2-cb63ce0dd405