Update README.md

update README to suggest the `.git` extension when pulling JSONAPI into a project via SPM.

Author: mattpolzin

README.md

@@ -66,7 +66,7 @@ If you find something wrong with this library and it isn't already mentioned und
 ### Swift Package Manager
 Just include the following in your package's dependencies and add `JSONAPI` to the dependencies for any of your targets.
 ```swift
-.package(url: "https://github.com/mattpolzin/JSONAPI", from: "4.0.0")
+.package(url: "https://github.com/mattpolzin/JSONAPI.git", from: "4.0.0")
 ```
 
 ### Xcode project