Update possessive usage by adding missing word "ending"

Clarified the exceptions for possessive forms of ancient proper names.

Author: Triquetra

skills/writing-clearly-and-concisely/elements-of-style/02-elementary-rules-of-usage.md

@@ -12,7 +12,7 @@ the witch's malice
 
 This is the usage of the United States Government Printing Office and of the Oxford University Press.
 
-Exceptions are the possessive of ancient proper names in *-es* and *-is*, the possessive *Jesus'*, and such forms as *for conscience' sake*, *for righteousness' sake*. But such forms as *Achilles' heel*, *Moses' laws*, *Isis' temple* are commonly replaced by
+Exceptions are the possessive of ancient proper names ending in *-es* and *-is*, the possessive *Jesus'*, and such forms as *for conscience' sake*, *for righteousness' sake*. But such forms as *Achilles' heel*, *Moses' laws*, *Isis' temple* are commonly replaced by
 
 the heel of Achilles