printf.3: Fix description of %a hexfloat output

The description of hexadecimal floating-point output is missing a character describing the exponent. The guarantee of at least one digit in the exponent is present in both C99 and POSIX.

man3/printf.3 +3 -1

index 18000bc70..d3510f7df 100644
@@ -730,7 +730,7 @@ conversion, the
  .I double
  argument is converted to hexadecimal notation (using the letters abcdef)
  in the style
-.RB [\-] 0x h \&. hhhh p \(+-;
+.RB [\-] 0x h \&. hhhh p \(+-d;
  for
  .B A
  conversion the prefix

@@ -746,6 +746,8 @@ and otherwise is sufficiently large to distinguish values of type
  .IR double .
  The digit before the decimal point is unspecified for nonnormalized
  numbers, and nonzero but otherwise unspecified for normalized numbers.
+The exponent always contains at least one
+digit; if the value is zero, the exponent is 0.
  .TP
  .B c
  If no