Update doc/src/challenges/0022-str-iter.md

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

Author: thanhnguyen-aws

doc/src/challenges/0022-str-iter.md

@@ -48,7 +48,7 @@ Prove the safety of the following safe functions that contain unsafe code:
 |next_back| MatchesInternal|
 |remainder| SplitAsciiWhitespace|
 
-Write and prove the safety contract for this unsafe function: __iterator_get_unchecked
+Write and prove the safety contract for this unsafe function `__iterator_get_unchecked`
 
 The verification must be unbounded---it must hold for str of arbitrary length.