docs: update backend-traffic-policy.md(remove targetRef.namespace)

[sunerpy]

As of version v1.3.1, BackendTrafficPolicy.spec.targetRef does not contain any namespace field.

What type of PR is this?

• "fix: fix docs"

What this PR does / why we need it:
As of version v1.3.1, BackendTrafficPolicy.spec.targetRef does not contain a namespace field.
This PR updates the documentation and API references to clarify this behavior.

Release Notes: No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.31%. Comparing base (8b74d01) to head (3704808).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5454   +/-   ##
=======================================
  Coverage   65.30%   65.31%           
=======================================
  Files         213      213           
  Lines       33915    33915           
=======================================
+ Hits        22147    22150    +3     
+ Misses      10438    10433    -5     
- Partials     1330     1332    +2     

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