fixed typo: term should be terms

two terms (input lifetime and output lifetime) so "term" needs to be plural.

Author: jonmarkprice

src/doc/book/lifetimes.md

@@ -290,7 +290,7 @@ lifetime parameters using three easily memorizable and unambiguous rules. This m
 acts as a shorthand for writing an item signature, while not hiding
 away the actual types involved as full local inference would if applied to it.
 
-When talking about lifetime elision, we use the term *input lifetime* and
+When talking about lifetime elision, we use the terms *input lifetime* and
 *output lifetime*. An *input lifetime* is a lifetime associated with a parameter
 of a function, and an *output lifetime* is a lifetime associated with the return
 value of a function. For example, this function has an input lifetime: