Update documentation/plain_rules.md

Co-Authored-By: Fermín Galán Márquez <fgalan@users.noreply.github.com>

documentation/plain_rules.md

@@ -279,7 +279,7 @@ NGSIv2 example:
 ```
 
 When using NGSIv2 in the update actions, the value field perform [string substitution](#string-substitution-syntax). If
-`value` is a String, Perseo will try parse the value to know real value and finally Perseo will parse the value taking into account the `type` field, this only applies to _`Number`_,
+`value` is a String, Perseo will try cast value to number, boolean or null (without paying attention to the attribute type). If the casting fails then String is used.
 _`Boolean`_ and _`None`_ types.
 
 **Data Types for NGSIv2:**