Improvements to wording of substr function documentation

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

website/docs/language/functions/substr.mdx

@@ -37,7 +37,7 @@ string after the given offset will be returned.
 world
 ```
 
-The length may be greater than the length of the string, in which case the substring
+If the length is greater than the length of the string, the substring
 will be the length of all remaining characters.
 
 ```