Update manifest.md

Suggestion for text when the edition isn't defined at all

Author: gauntface

src/doc/src/reference/manifest.md

@@ -125,15 +125,14 @@ set to the latest edition. Setting the `edition` key in
 `[package]` will affect all targets/crates in the package, including test
 suites, benchmarks, binaries, examples, etc.
 
-If the `edition` key is not set to a specific [Rust Edition] in your
-`Cargo.toml`, Cargo will default to 2018.
-
 ```toml
 [package]
 # ...
 edition = '2018'
 ```
 
+If the `edition` key is not set in your `Cargo.toml` file, Cargo will default to 2015.
+
 #### The `description` field
 
 The description is a short blurb about the package. [crates.io] will display