Make base-55 encoding & decoding match

Changed "U"'s encoding to -210, so now when the README claims "U =
-210", it's actually accurate :)

src/base_55.rs

@@ -5,7 +5,7 @@ pub fn char2num(character: char) -> i32 {
         'X' => -24,
         'W' => -23,
         'V' => -22,
-        'U' => -21,
+        'U' => -210,
         'T' => -20,
         'R' => -18,
         'S' => -19,