update dep

Satoshi-Kusumoto · cramertj · commit 15281c6a3d51 · 2020-02-03T09:45:20.000-08:00
diff --git a/src/02_execution/04_executor.md b/src/02_execution/04_executor.md
@@ -27,7 +27,7 @@ authors = ["XYZ Author"]
 edition = "2018"
 
 [dependencies]
-futures-preview = "=0.3.0-alpha.17"
+futures = "0.3"
 ```
 
 Next, we need the following imports at the top of `src/main.rs`:
