sh.1: printf %d: fix precision/output base documentation

@vinc17fr writes:
> For printf with %d, when a single dot is used without a
> precision, the precision is taken as 0. So the ksh93 specific
> case .. to provide a base without giving a precision (e.g. %..2d)
> is ambiguous: does this mean that the precision is missing (like
> without any dot, thus defaulting to 1) or does this mean that the
> precision is 0 (like with a single dot)? This should be
> documented in the man page (sh.1).
>
> A test shows that the precision is 1 in this case (which is much
> more useful for the user than 0):
>
>	$ printf ">%..2d<\n" 0
>	>0<

Resolves: #679

src/cmd/ksh93/sh.1

@@ -7289,6 +7289,9 @@ In this case, the
 flag character causes
 .IB base #
 to be prepended.
+When an output base is specified without giving a precision (e.g.
+.BR %..2d ),
+the precision defaults to 1 instead of 0.
 .TP
 .B #
 The