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

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

* NEWS: "an misleading"
* src/expr.c: "a integer
* src/ptx.c (find_occurs_in_text): "a end"
* src/shred.c (do_wipefd): "a infinite"
* src/sort.c (SUBTHREAD_LINES_HEURISTIC): "an dual-core"
(compare_random): "an checksum"
* cfg.mk (old_NEWS_hash): Update, since the typo was in old news.