[Ruleset Engine] Fix field name (#20868)

Author: pedrosousa

src/content/docs/ruleset-engine/rules-language/expressions/index.mdx

@@ -41,7 +41,7 @@ Where:
 For example, this expression uses the `and` operator to target requests to `www.example.com` that are not on ports 80 or 443:
 
 ```txt
-host eq www.example.com and not cf.edge.server_port in {80 443}
+http.host eq www.example.com and not cf.edge.server_port in {80 443}
 ```
 
 Compound expressions have the following general syntax: