Update build-lambda-zip.yml

remove branch name from `go get -u` line, to match README instructions

Author: bmoffatt

.github/workflows/build-lambda-zip.yml

@@ -26,4 +26,4 @@ jobs:
           GO111MODULE: on
         run: |
           go env
-          go get -u github.com/aws/aws-lambda-go/cmd/build-lambda-zip@master
+          go get -u github.com/aws/aws-lambda-go/cmd/build-lambda-zip