Align string type for edition (#4197)

Author: BO41

Configurations.md

@@ -519,8 +519,8 @@ fn main() {
 
 Specifies which edition is used by the parser.
 
-- **Default value**: `2015`
-- **Possible values**: `2015`, `2018`
+- **Default value**: `"2015"`
+- **Possible values**: `"2015"`, `"2018"`
 - **Stable**: Yes
 
 Rustfmt is able to pick up the edition used by reading the `Cargo.toml` file if executed
@@ -2621,4 +2621,4 @@ Break comments to fit on the line
 // magna aliqua. Ut enim ad minim veniam, quis nostrud
 // exercitation ullamco laboris nisi ut aliquip ex ea
 // commodo consequat.
-```
+```