fix Minisat 2.2.1 URL

Author: Daniel Kroening

COMPILING

@@ -142,7 +142,7 @@ Follow these instructions:
 1) You need a SAT solver (in source). We recommend MiniSat2. Using a
    browser, download from
 
-   http://minisat.se/downloads/minisat-2.2.1.tar.gz
+   http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
 
    and then unpack with