maint: replace misused "a" with "an"

maint: replace misused "a" with "an"

* doc/intprops.texi: "a integer"
* doc/regex.texi: "a explanation"
* lib/alignof.h: "a object"
* lib/argmatch.h: "a explanation"
* lib/argp-help.c: "a option" and "a OPTION_DOC"
* lib/stdint.in.h: "a integer"
* lib/userspec.c: "a owner"
* doc/gnulib.texi: Fix "a idea", and reword.
(cherry picked from commit 93d7550462f21202b69712f3d489399963965ddd)