Update initialization instruction

[emilychu9318]

Closes #[issue number]

What this PR does / why we need it:
When using VS Code devcontainer plugin to setup ASOv2 dev environment, I am running into following error when task defult: task: Command "./scripts/build-version.py v2" failed: exit status 1
This is because the repository was cloned with a depth setting, which create a shallow copy.
The fix is to convert the shallow clone to regular one by: git fetch --unshallow

How does this PR make you feel:

If applicable:

• this PR contains documentation

[matthchr]

had one minor comment but LGTM other than that. If you think that suggested language is helpful please update the PR and we can run CI and merge.

Thanks for this contribution btw!

[theunrepentantgeek]

/ok-to-test sha=41abfbe

[codecov-commenter]

Codecov Report

Merging #2781 (41abfbe) into main (11d3f62) will decrease coverage by 0.02%.
The diff coverage is 62.41%.

@@            Coverage Diff             @@
##             main    #2781      +/-   ##
==========================================
- Coverage   56.01%   56.00%   -0.02%     
==========================================
  Files         978      979       +1     
  Lines      348311   348290      -21     
==========================================
- Hits       195115   195046      -69     
- Misses     122363   122409      +46     
- Partials    30833    30835       +2     

Impacted Files	Coverage Δ
...zations/namespaces_topic_subscription_extension.go	0.00% <0.00%> (ø)
v2/internal/config/vars.go	68.42% <ø> (+1.41%)	⬆️
...peline/apply_defaulter_and_validator_interfaces.go	22.92% <0.00%> (+0.43%)	⬆️
...ator/internal/config/object_model_configuration.go	71.10% <0.00%> (-3.57%)	⬇️
...ls/generator/internal/config/type_configuration.go	60.64% <0.00%> (-5.52%)	⬇️
.../internal/functions/index_registration_function.go	0.00% <0.00%> (ø)
.../internal/functions/locatable_resource_function.go	0.00% <0.00%> (ø)
...ternal/functions/one_of_json_unmarshal_function.go	0.00% <0.00%> (ø)
...nal/functions/kubernetes_admissions_validations.go	64.75% <83.33%> (-0.37%)	⬇️
v2/internal/reconcilers/arm/arm_client_cache.go	69.76% <100.00%> (+0.47%)	⬆️
... and 23 more

... and 1 file with indirect coverage changes

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[matthchr]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).