Fix attachInterface documentation typo

[logan]

Summary

This commit replaces registerInterface with attachInterface.

mlir/docs/Interfaces.md

struct ExternalModelExample
   // No need to define `exampleInterfaceHook` that has a default implementation
   // in `ExternalModel`. But it can be overridden if desired.
 }
 
 int main() {
   MLIRContext context;
   /* ... */;
 
-  // Register the interface model with the type in the given context before
-  // using it. The dialect contaiing the type is expected to have been loaded
+  // Attach the interface model to the type in the given context before
+  // using it. The dialect containing the type is expected to have been loaded
   // at this point.
-  IntegerType::registerInterface<ExternalModelExample>(context);
+  IntegerType::attachInterface<ExternalModelExample>(context);
 }
 ```
 
 Note: It is strongly encouraged to only use this mechanism if you "own" the
 interface being externally applied. This prevents a situation where neither the
 owner of the dialect containing the object nor the owner of the interface are
 aware of an interface implementation, which can lead to duplicate or
 diverging implementations.