Use e2e.test binary instead of test/e2e in example invocations

[wongma7]

What type of PR is this?
/kind documentation

What this PR does / why we need it: the example invocation should use the binary instead of ginkgo because ideally the reader will only need to download the e2e.test binary and not need to clone the repo and/or have ginkgo in their PATH
test/e2e assumes the user has cloned k/k and is working inside it, we want to document the case where the user has downloaded the binary and doesn't need to wait for anything to build

/sig storage

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[wongma7]

/cc @pohly
/cc @msau42
PTAL, thank you!!

[msau42]

One downside is that we lose the parallelism. When I spoke with test-infra, they said that ginkgo -p flag will invoke the e2e binary multiple times with different -ginkgo.prallel.node values to manage the parallelism.

So I think we can just replace "./test/e2e" with the e2e binary in the original command

[wongma7]

I see, done

[pohly]

My original intent was to keep this README.md generic, i.e. not assume that this package is used only in the Kubernetes e2e.test binary. But I guess anyone who does that is able to figure out how to generate an e2e.test binary or invoke ginkgo on the source, so this change is fine.

[msau42]

/lgtm
/approve

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