Fix typo in metavariable-name example (#2601)

In this example metavariable-regex was accidentally used instead of metavariable-name.

Author: jkinsfather

docs/writing-rules/rule-syntax.md

@@ -816,7 +816,7 @@ rules:
   - id: insecure-method
     patterns:
       - pattern: $MODULE.method(...)
-      - metavariable-regex:
+      - metavariable-name:
           metavariable: $MODULE
           modules:
            - foo