stdio-common/printf-prs.c: Reword comment

'this' can be understood as the current parameter, but in this case it
is meaning the other one, the one holding the width/precission.

'it' better describes that parameter, differentiating it from the
one corresponding to the current specifier.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>