Replace version specification in README with "desired version".

README.md

@@ -22,7 +22,7 @@ Add `evalexpr` as dependency to your `Cargo.toml`:
 
 ```toml
 [dependencies]
-evalexpr = "7"
+evalexpr = "<desired version>"
 ```
 
 Then you can use `evalexpr` to **evaluate expressions** like this:

src/lib.rs

@@ -5,7 +5,7 @@
 //!
 //! ```toml
 //! [dependencies]
-//! evalexpr = "7"
+//! evalexpr = "<desired version>"
 //! ```
 //!
 //! Then you can use `evalexpr` to **evaluate expressions** like this: