Add explanation for auth_strength calculation

Clarify the calculation of auth_strength in the specification.

Author: char0n

docs/specification/spec.md

@@ -731,6 +731,7 @@ It evaluates the average strength of all security schemes using normative scores
 ```text
 auth_strength = sum(strength_scores) / schemes_count
 ```
+
 Where
 - `strength_scores` is the list of calculated scheme strengths
 - `schemes_count` is the number of defined schemes