fix: Correct guild member banner URL

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
See https://discord.com/developers/docs/reference#image-formatting-cdn-endpoints.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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Name	Status	Preview	Comments	Updated (UTC)
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.96%. Comparing base (65883f3) to head (f8706e0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10677      +/-   ##
==========================================
+ Coverage   37.91%   37.96%   +0.04%     
==========================================
  Files         239      239              
  Lines       14877    14877              
  Branches     1376     1377       +1     
==========================================
+ Hits         5641     5648       +7     
+ Misses       9225     9218       -7     
  Partials       11       11              

Flag	Coverage Δ
proxy	64.63% <ø> (ø)
rest	87.47% <100.00%> (+0.66%)	⬆️
ws	34.45% <ø> (ø)

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