[mlir][emitc] Fix invalid syntax in example of emitc.return (llvm#121112)

A return type of `emitc.func` must be specified with `->` instead of
`:`. I've verified the syntax using `mlir-translate --mlir-to-cpp`.

Author: hatoo

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

@@ -727,7 +727,7 @@ def EmitC_ReturnOp : EmitC_Op<"return", [Pure, HasParent<"FuncOp">,
     Example:
 
     ```mlir
-    emitc.func @foo() : (i32) {
+    emitc.func @foo() -> (i32) {
       ...
       emitc.return %0 : i32
     }