docs: fix extension-server instructions

[Dean-Coakley]

What this PR does / why we need it:
Updates instructions to match where extension-server config is now loaded from.

Which issue(s) this PR fixes:

Fixes #4811

Release Notes: No

[Dean-Coakley]

@zhaohuabing

1. Should the fix for this look something like this? I had a look around but I don't see a reference in the code for the expected format in the configmap.
   These changes are a guess based on other docs such as https://github.com/envoyproxy/gateway/blob/main/site/content/en/docs/tasks/observability/gateway-observability.md#metrics

2. Should the same fix be copied over to versioned docs v1.1 or are the current docs correct?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.76%. Comparing base (6466ceb) to head (81eec0c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5013    +/-   ##
========================================
  Coverage   66.76%   66.76%            
========================================
  Files         209      209            
  Lines       32255    32358   +103     
========================================
+ Hits        21534    21604    +70     
- Misses       9427     9451    +24     
- Partials     1294     1303     +9     

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[liorokman]

LGTM