S6418/php: fix typo in variable name

`description.adoc` is using `detections` and not `detectons` as a variable:
```
This rule detects {detections} having a name matching a list of words (secret, token, credential, auth, api[_.-]?key) being assigned a pseudorandom hard-coded value.
```

This PR makes the value rendered correctly on https://sonarsource.github.io/rspec/#/rspec/S6418/php

rules/S6418/php/rule.adoc

@@ -1,4 +1,4 @@
-:detectson: variables/fields
+:detections: variables/fields
 :defaultsensibility: 5
 
 include::../description.adoc[]