docs: Update HTTPRoute status example

[jonstacks]

What type of PR is this?
/kind documentation

What this PR does / why we need it:

From looking at the API types, it looks like RouteParentStatus.ParentRef's json struct tag is singular, not plural:

gateway-api/apis/v1/shared_types.go

Lines 436 to 439 in 1e14dd6

	type RouteParentStatus struct {
	// ParentRef corresponds with a ParentRef in the spec that this
	// RouteParentStatus struct describes the status of.
	ParentRef ParentReference `json:"parentRef"`

This just updates the docs to match.

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

NONE

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[youngnick]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jonstacks, youngnick

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[youngnick]

/ok-to-test