[mlir][vector] Update document for vector.splat(NFC)

This PR updates the document for `vector.splat`, specifying that the
operand type must match the element type of the result.

Author: CoTinker

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -2807,8 +2807,8 @@ def Vector_SplatOp : Vector_Op<"splat", [
     Example:
 
     ```mlir
-    %s = arith.constant 10.1 : f32
-    %t = vector.splat %s : vector<8x16xi32>
+    %s = arith.constant 1.0 : f32
+    %t = vector.splat %s : vector<8x16xf32>
     ```
   }];