[mlir][OpFormatGen] Fix incorrect kind used for RegionsDirective

[rriddle]

Summary

I attempted to write a test case for this, but the situations in which the kind is used for RegionDirective and ResultsDirective have zero overlap; meaning that there isn't a situation in which sharing the kind creates a conflict.

mlir/tools/mlir-tblgen/OpFormatGen.cpp

public:
   static bool classof(const Element *ele) { return ele->getKind() == type; }
 };
 /// This class represents the `operands` directive. This directive represents
 /// all of the operands of an operation.
 using OperandsDirective = DirectiveElement<Element::Kind::OperandsDirective>;
 
 /// This class represents the `regions` directive. This directive represents
 /// all of the regions of an operation.
-using RegionsDirective = DirectiveElement<Element::Kind::ResultsDirective>;
+using RegionsDirective = DirectiveElement<Element::Kind::RegionsDirective>;
 
 /// This class represents the `results` directive. This directive represents
 /// all of the results of an operation.
 using ResultsDirective = DirectiveElement<Element::Kind::ResultsDirective>;
 
 /// This class represents the `successors` directive. This directive represents
 /// all of the successors of an operation.
 using SuccessorsDirective =