help-opal-shmem-mmap.txt: trivial typo fix

(cherry picked from commit 5207429734d131f80397c7bdab12f52de05881a2)
Merge pull request #54 from jsquyres/pr/1.8-trivial-help-msg-typo-fix

help-opal-shmem-mmap.txt: trivial typo fix