fix: add modules /v3 suffix to install example

The previous example in the README installed 1.2.8 rather than the
latest 3.x series

Author: dnwe

README.md

@@ -28,7 +28,7 @@ make something configurable!
 You can do that by using:
 
 ```sh
-go get -u github.com/bombsimon/wsl/cmd/...
+go get -u github.com/bombsimon/wsl/v3/cmd/...
 ```
 
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