fix deps in gtest example (cpp-use-cases.md)

[cmachida]

reference:

https://google.github.io/googletest/quickstart-bazel.html

cc_test(
    name = "hello_test",
    size = "small",
    srcs = ["hello_test.cc"],
    deps = [
        "@googletest//:gtest",
        "@googletest//:gtest_main",
    ],
)