libcacard: Spelling and grammar fixes in documentation

* it's -> its

* it's -> it is (that's no fix, but makes future checks easier)

* this functions -> this function

* replacable -> replaceable

* reader's -> readers

* logins into -> logs into

v2:
Also replace 'aid' by 'AID' (thanks to Peter Maydell for this hint).

v3:
Fix sentence (contributed by Alon Levy / Robert Relyea).

Cc: Alon Levy <alevy@redhat.com>
Cc: Robert Relyea <rrelyea@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
