Update functions-and-operators/string-functions.md

Co-authored-by: xixirangrang <[email protected]>

functions-and-operators/string-functions.md

@@ -1329,7 +1329,7 @@ Return the starting index of the substring that matches the regular expression (
 
 The `REGEXP_INSTR(str, regexp, [start, [match, [ret, [match_type]]]])` function returns the position of the match if the regular expression (`regexp`) matches the string (`str`).
 
-If either the `str` or `regexp` is NULL then the function returns NULL.
+If either the `str` or `regexp` is `NULL`, then the function returns `NULL`.
 
 Examples: