[mlir] Update comment in Region.h

The file in which `Region::viewGraph` is defined has changed. This should have been updated with D106342.

Differential Revision: https://reviews.llvm.org/D107517

Author: matthias-springer

mlir/include/mlir/IR/Region.h

@@ -296,7 +296,7 @@ class Region {
 
   /// Displays the CFG in a window. This is for use from the debugger and
   /// depends on Graphviz to generate the graph.
-  /// This function is defined in ViewRegionGraph and only works with that
+  /// This function is defined in ViewOpGraph.cpp and only works with that
   /// target linked.
   void viewGraph(const Twine &regionName);
   void viewGraph();