Fix incorrect example in installation README

[PaulCapestany]

Currently there is no stable overlay available, example in README should use stable-master instead.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design

/kind documentation

/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Fixes incorrect documentation for installing the driver.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:
NONE

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[k8s-ci-robot]

Hi @PaulCapestany. Thanks for your PR.

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[leiyiz]

/ok-to-test

[leiyiz]

/lgtm
/approve

[leiyiz]

/assign @saikat-royc

[leiyiz]

/test pull-gcp-compute-persistent-disk-csi-driver-sanity

[judemars]

Hey there! Just letting you know that I have just fixed and re-enabled the sanity tests for PD in go/pdcsi-oss-driver/issues/990 so if you experience failures in your sanity test PR gate build because you're on an old branch before the tests were fixed, merging the latest commits from master into this branch it should make them pass. Let me know if you have any questions!

[msau42]

/retest

[msau42]

/release-note-none
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: leiyiz, msau42, PaulCapestany

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