fix: field name error for monitor-grafana-secret example.

[ben-wangz]

What problem does this PR solve?

fix the field name error for monitor-grafana-secret example.

What is changed and how does it work?

according to https://github.com/pingcap/tidb-operator/blob/master/pkg/apis/pingcap/v1alpha1/tidbmonitor_types.go#L273 (line 273 and 277), the field names should be usernameSecret and passwordSecret

Code changes

• Has Go code change
• [ x] Has CI related scripts change

Tests

• Unit test
• E2E test
• [x ] Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

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[sre-bot]

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[ti-chi-bot]

Welcome @ben-wangz! It looks like this is your first PR to pingcap/tidb-operator 🎉