Add a simple example for execution order (#1992)

The text refers to "the example program above", but said example is
actually near the top of the file (thousands of lines above). The
example is also longer than necessary for the purposes of illustrating
execution order.

docs/spec.md

@@ -5960,9 +5960,18 @@ The execution order is implementation-defined as long as it is aligned with
 dataflow, i.e. if ops are executed before their uses. In StableHLO, all
 side-effecting ops consume one token and produce one token (multiple tokens can
 be multiplexed into one token via `after_all`), so the execution order of side
-effects is also aligned with dataflow. Possible execution orders of the example
-program above are `%0` → `%1` → `%2` → `%3` → `%4` → `return` or `%3` → `%0` →
-`%1` → `%2` → `%4` → `return`.
+effects is also aligned with dataflow. For example, in the below program
+there are two possible execution orders: `%0` → `%1` → `%2` → `return` and
+`%1` → `%0` → `%2` → `return`.
+
+```mlir
+func.func @main() -> tensor<f64> {
+  %0 = stablehlo.constant dense<1.0> : tensor<f64>
+  %1 = stablehlo.constant dense<2.0> : tensor<f64>
+  %2 = stablehlo.add %0, %1 : tensor<f64>
+  return %2 : tensor<f64>
+}
+```
 
 More formally, a **StableHLO process** is a combination of:
 1\) a StableHLO program, 2) operation statuses (not executed yet,