[mlir][doc] Fix typo in Ch6 tutorial: 'toy.cpp' → 'toyc.cpp' (NFC) (llvm#161245)

This patch fixes a documentation typo in the MLIR Toy tutorial (Ch6).
The tutorial currently refers to `examples/toy/Ch6/toy.cpp`, but the
correct
file name is `examples/toy/Ch6/toyc.cpp`.

Author: josephbak

mlir/docs/Tutorials/Toy/Ch-6.md

@@ -245,7 +245,7 @@ define void @main()
 ```
 
 The full code listing for dumping LLVM IR can be found in
-`examples/toy/Ch6/toy.cpp` in the `dumpLLVMIR()` function:
+`examples/toy/Ch6/toyc.cpp` in the `dumpLLVMIR()` function:
 
 ```c++