Change man page to reflect current behavior of primes

Notes:
Fri Feb 23 14:01:03 EST 2007 geoff
    i decided to make the code match the manual
    since the manual describes more useful behaviour.


Reference: /n/sources/patch/sorry/primes-man
Date: Tue Feb 20 03:07:24 CET 2007
Signed-off-by: lawler@buffalo.edu
Reviewed-by: geoff

--- /sys/man/1/factor	Tue Feb 20 03:04:59 2007
+++ /sys/man/1/factor	Tue Feb 20 03:04:58 2007
@@ -53,11 +53,7 @@
 .I finish
 is missing,
 .I primes
-prints without end;
-if
-.I start
-is missing, it reads the starting number from the
-standard input.
+prints one prime number.
 .SH SOURCE
 .B /sys/src/cmd/factor.c
 .br