cal: fix first week calculation

cal: fix first week calculation

Commit efafeaf set 1 Jan as week 1, but the change
was missed in week_to_day() and in the man page.

Before
cal --week=40 --iso 1752
      October 1752
   Su Mo Tu We Th Fr Sa
41  1  2  3  4  5  6  7
42  8  9 10 11 12 13 14
43 15 16 17 18 19 20 21
44 22 23 24 25 26 27 28
45 29 30 31

Patched
cal --week=40 --iso 1752
     September 1752
   Su Mo Tu We Th Fr Sa
36                 1  2
37  3  4  5  6  7  8  9
38 10 11 12 13 14 15 16
39 17 18 19 20 21 22 23
40 24 25 26 27 28 29 30

Signed-off-by: J William Piggott <elseifthen@gmx.com>