docs: Fixed the key for headers in prometheus based argo analysis

[Leegin-darknight]

The existing key "name" results in an error. The expected key is "key" as defined in the struct:

argo-rollouts/pkg/apis/rollouts/v1alpha1/analysis_types.go

Line 567 in 99ae20c

Key string `json:"key" protobuf:"bytes,1,opt,name=key"`

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[github-actions]

Go Published Test Results

2 093 tests   2 093 ✅  2m 49s ⏱️
  118 suites      0 💤
    1 files        0 ❌

Results for commit 74a1bed.

[github-actions]

E2E Tests Published Test Results

  4 files    4 suites   3h 25m 50s ⏱️
106 tests  98 ✅  6 💤 2 ❌
428 runs  400 ✅ 24 💤 4 ❌

For more details on these failures, see this check.

Results for commit 74a1bed.

[leoluz]

LGTM