[MAN] Remove spurious space in descriptions of PS1, PS2, PS4

From: Kalle Olavi Niemitalo <kon@iki.fi>

LANG=C man dash shows:

     PS1        The primary prompt string, which defaults to ``$  '', unless
                you are the superuser, in which case it defaults to ``#  ''.

     PS2        The secondary prompt string, which defaults to ``>  ''.

     PS4        Output before each line when execution trace (set -x) is
                enabled, defaults to ``+  ''.

Each of the documented default values has a graphic character and
two spaces between the quotation marks.  However, the actual
default values have only one space, rather than two.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>