docs: update gpback installation steps according to operations

[TomShawn]

docs: update gpback installation steps according to operations

I find the steps for building gpbackup confusing. So I modify the steps according to my own operation. By the modified steps, I can successfully build gpback.

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[TomShawn]

@tuhaihe PTAL

[tuhaihe]

LGTM.

[tuhaihe]

Hi @Terry1504 Please help review this PR. Thanks!