Replace "msecs" with "ms" in all comments and messages

Replace "msecs" with "ms" in all comments and messages

* README.md (Bugs): Replace "msecs" with "ms".
* doc/README.environment (GC_PAUSE_TIME_TARGET): Likewise.
* doc/gcdescr.md (Generational Collection and Dirty Bits): Likewise.
* alloc.c [!NO_CLOCK] (full_gc_total_time): Replace "msecs" with "ms"
in comment.
* cord/tests/de.c [WIN32] (beep): Likewise.
* pthread_support.c [USE_SPIN_LOCK] (GC_lock): Likewise.
* alloc.c [!GC_DISABLE_INCREMENTAL && !NO_CLOCK]
(GC_timeout_stop_func): Replace "msecs" with "ms" in the printed
string.
* alloc.c [!NO_CLOCK] (GC_try_to_collect_inner, GC_stopped_mark,
GC_finish_collection): Likewise.
* misc.c [!NO_CLOCK] (GC_dump_named): Likewise.
* reclaim.c [!NO_CLOCK] (GC_reclaim_all): Likewise.
* tests/test.c [!NO_CLOCK] (check_heap_stats): Likewise.