[fix] replace ${go env GOPATH} with $(go env GOPATH) to get the prope…

[double12gzh]

What this PR does / why we need it:
update shell command in dev doc.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #1951

Checklist:

• Docs included if any changes are user facing

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[tenzen-y]

Thanks!
/lgtm
/approve

/hold for CI
@kubeflow/wg-training-leads Could you approve CI?

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: double12gzh, tenzen-y

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[andreyvelich]

Thank you for the update @double12gzh!
/hold cancel

[coveralls]

Pull Request Test Coverage Report for Build 7002135060

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.08%) to 42.792%

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Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob.go	1	91.06%
pkg/controller.v1/mpi/mpijob_controller.go	6	80.48%

Totals
Change from base Build 6969213394:	-0.08%
Covered Lines:	3746
Relevant Lines:	8754

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💛 - Coveralls