Correct a typo on the async-std version (#508)

Correct a typo on the async-std version in the Cargo.toml file of the documentation.

docs/src/tutorial/specification.md

@@ -51,5 +51,5 @@ Add the following lines to `Cargo.toml`:
 ```toml
 [dependencies]
 futures = "0.3.0"
-async-std = "1.00"
+async-std = "1.0.0"
 ```