build: add example usage in cmd/main.go and correct build script

.github/workflows/go.yml

@@ -13,10 +13,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.13
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.13
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory