By Jennifer Piacenti The defending Super Bowl champs are 2.5-point underdogs in San Francisco. The San Francisco 49ers (1-2) host the division-leading Los Angeles Rams (2-1) in an NFC West showdown on Monday Night Football. Both teams enter this matchup 1-2 against the spread for the season, with the Rams winning two games straight up. San Francisco has won four of the last five matchups between these two teams, so it seems safe to say Kyle Shanahan has Sean McVay’s number. The Rams enter this one as the slight road dogs, despite their better record. Here is where the line stands at SI Sportsb…