fix typo

Author: andylokandy

docs/doc/30-reference/20-functions/40-string-functions/substring.md

@@ -24,7 +24,7 @@ SUBSTRING(str FROM pos FOR len)
 | Arguments   | Description |
 | ----------- | ----------- |
 | str | The main string from where the character to be extracted |
-| pos | The position (starting from 1) the substring starting at. If negative, counts from the end |
+| pos | The position (starting from 1) the substring to start at. If negative, counts from the end |
 | len | The maximun length of the substring to extract |
 
 ## Return Type