maint: correct misuse of "a" and "an"

maint: correct misuse of "a" and "an"

* doc/regex.texi (Collating Symbol Operators): s/an close.../a close/
* lib/argp-help.c: "an docum...": s/an/a/
* lib/argp-parse.c: "An vector": s/An/A/
* lib/execute.c: "an native": s/an/a/
* lib/spawn-pipe.c: Likewise.
* lib/gc.h: "an Gc_rc": s/an/a/
* lib/unigbrk.in.h: "an grapheme": s/an/a/
* lib/fts.c: "an stat.st_dev": s/an/a/