Fix up wording in lifetime elision docs

Fixes #25915

Author: steveklabnik

src/doc/trpl/lifetimes.md

@@ -305,7 +305,7 @@ fn substr<'a>(s: &'a str, until: u32) -> &'a str; // expanded
 fn get_str() -> &str; // ILLEGAL, no inputs
 
 fn frob(s: &str, t: &str) -> &str; // ILLEGAL, two inputs
-fn frob<'a, 'b>(s: &'a str, t: &'b str) -> &str; // Expanded: Output lifetime is unclear
+fn frob<'a, 'b>(s: &'a str, t: &'b str) -> &str; // Expanded: Output lifetime is ambiguous
 
 fn get_mut(&mut self) -> &mut T; // elided
 fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded