Fix wording in introduction.md (#397)

exercises/concept/log-parser/.docs/introduction.md

@@ -18,7 +18,7 @@ The most common way to create regular expressions is using the `regex.from_strin
 let assert Ok(re) = regex.from_string("test")
 ```
 
-The regular expression creation functions an error if the regular expression syntax is invalid, so a let-assertion has been used here to ensure the regular expression is valid.
+The regular expression creation functions return an error if the regular expression syntax is invalid, so a let-assertion has been used here to ensure the regular expression is valid.
 
 The `regex.check` function can be used to check if a regular expression matches a string.