Provide prescriptive versions in example

Author: frankmcsherry

mdbook/src/chapter_0/chapter_0_0.md

@@ -25,8 +25,8 @@ version = "0.1.0"
 authors = ["Your Name <[email protected]>"]
 
 [dependencies]
-timely = { git = "https://github.com/TimelyDataflow/timely-dataflow" }
-differential-dataflow = { git = "https://github.com/TimelyDataflow/differential-dataflow" }
+timely = "=0.13.0"
+differential-dataflow = "=0.13.0"
 ```
 
 You should only need to add those last two lines there, which bring in dependencies on both [timely dataflow](https://github.com/TimelyDataflow/timely-dataflow) and [differential dataflow](https://github.com/TimelyDataflow/differential-dataflow). We will be using both of those.