Fix punctuation.

crawl-ref/source/dat/descript/monsters.txt

@@ -2216,7 +2216,7 @@ polterguardian
 Many poltergeists are fiercely territorial over the domains they choose to
 haunt, but these have extended that protection even to the people who share
 their domain with them. Some say they must be the lingering souls of those who
-guarded the Vaults in life, others suggest the Ironbound Order simply recruited
+guarded the Vaults in life; others suggest the Ironbound Order simply recruited
 them in common cause. Whichever is true, their very presence shelters other
 creatures from missiles, causing sling and spell alike to veer off-course at the
 last moment.