fix(processors): fix requirements interface example (#35)

Fixes an incorrect code block which shows an interface `implementing`
the Requirements interface, where it should be `extends`.

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docs/processors/requirements.md

@@ -18,7 +18,7 @@ by the generic types. Example:
 
 ```java
 public interface YourRequirementInterface
-  implements Requirement<YourSenderType, YourRequirementInterface> {
+  extends Requirement<YourSenderType, YourRequirementInterface> {
 
     // Example method
     @NonNull String errorMessage();