Fix googletest target name

[ericriff]

I was following the tutorial and hit a wall here.

 bazel test //test:hello-test
ERROR: /home/eriff/.cache/bazel/_bazel_eriff/f6bb6ff4e6e38165bb9ceee09de6c8af/external/googletest+/BUILD.bazel: no such target '@@googletest+//:main': target 'main' not declared in package '' defined by /home/eriff/.cache/bazel/_bazel_eriff/f6bb6ff4e6e38165bb9ceee09de6c8af/external/googletest+/BUILD.bazel
ERROR: /home/eriff/repos/bazel-examples/cpp-tutorial/stage3/test/BUILD:1:8: no such target '@@googletest+//:main': target 'main' not declared in package '' defined by /home/eriff/.cache/bazel/_bazel_eriff/f6bb6ff4e6e38165bb9ceee09de6c8af/external/googletest+/BUILD.bazel and referenced by '//test:hello-test'
ERROR: Analysis of target '//test:hello-test' failed; build aborted: Analysis failed

Turns out the target name was wrong

[google-cla]

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