docs(GuildTextThreadManager): Document correct @extend

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
This was not documenting the correct class it was extending.

Status and versioning classification:

• This PR only includes non-code changes, like changes to documentation, README, etc.

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Name	Status	Preview	Updated
discord-js	✅ Ready (Inspect)	Visit Preview	Sep 19, 2022 at 7:49PM (UTC)

[codecov]

Codecov Report

Merging #8639 (88c1385) into main (c446a84) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 88c1385 differs from pull request most recent head fd7fc59. Consider uploading reports for the commit fd7fc59 to get more accurate results

@@            Coverage Diff             @@
##             main    #8639      +/-   ##
==========================================
+ Coverage   86.66%   86.68%   +0.01%     
==========================================
  Files          86       87       +1     
  Lines        8771     8784      +13     
  Branches     1105     1110       +5     
==========================================
+ Hits         7601     7614      +13     
  Misses       1128     1128              
  Partials       42       42              

Flag	Coverage Δ
builders	100.00% <ø> (ø)
collection	100.00% <ø> (ø)
proxy	74.10% <ø> (ø)
rest	91.99% <ø> (ø)
utilities	100.00% <ø> (?)
voice	63.86% <ø> (ø)
ws	60.47% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/actions/src/formatTag/formatTag.ts	100.00% <0.00%> (ø)

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