Update doc/src/challenges/0021-str-pattern-pt2.md

Co-authored-by: Carolyn Zech <cmzech@amazon.com>

Author: thanhnguyen-aws

doc/src/challenges/0021-str-pattern-pt2.md

@@ -12,7 +12,7 @@ Verify the safety of `StrSearcher` implementation in `str::pattern`.
 
 ## Motivation
 
-String and str types are widely used in Rust programs. Verifying Rust String and str functions in Rust standard library is important in ensuring the safety of these programs.
+String and `str` types are widely used in Rust programs, so it is important that their associated functions do not cause undefined behavior.
 
 ## Description