Fix missing +/- prefixes in migration diff for v1.39.0.md (#6975)

Improves readability when migrating to v1.39.0

Signed-off-by: Simon Shine <simon@simonshine.dk>

Author: sshine

website/content/en/docs/upgrading-sdk-version/v1.39.0.md

@@ -16,18 +16,18 @@ so this release should be easier to follow.
 
 2) [go/v4] Update your `go.mod` file to upgrade the dependencies and run `go mod tidy` to download them
    ```go
-    github.com/onsi/ginkgo/v2 v2.17.1
-    github.com/onsi/gomega v1.32.0
-    k8s.io/api v0.30.1
-    k8s.io/apimachinery v0.30.1
-    k8s.io/client-go v0.30.1
-    sigs.k8s.io/controller-runtime v0.18.4
-    github.com/onsi/ginkgo/v2 v2.19.0
-    github.com/onsi/gomega v1.33.1
-    k8s.io/api v0.31.0
-    k8s.io/apimachinery v0.31.0
-    k8s.io/client-go v0.31.0
-    sigs.k8s.io/controller-runtime v0.19.0
+   - github.com/onsi/ginkgo/v2 v2.17.1
+   - github.com/onsi/gomega v1.32.0
+   - k8s.io/api v0.30.1
+   - k8s.io/apimachinery v0.30.1
+   - k8s.io/client-go v0.30.1
+   - sigs.k8s.io/controller-runtime v0.18.4
+   + github.com/onsi/ginkgo/v2 v2.19.0
+   + github.com/onsi/gomega v1.33.1
+   + k8s.io/api v0.31.0
+   + k8s.io/apimachinery v0.31.0
+   + k8s.io/client-go v0.31.0
+   + sigs.k8s.io/controller-runtime v0.19.0
    ```
 
 3) [go/v4] Update your `Makefile` with the below changes: