Fix rejoining activity

[nicksellen]

What does this PR do?

Fixes bug when rejoining activity. The issue was a stray this reference, leftover from converting to vue3 setup style...

Links to related issues

Checklist

• [ ] added a test, or explain why one is not needed/possible...
• [ ] no unrelated changes
• [ ] asked someone for a code review
• [ ] joined #karrot:matrix.org
• [ ] added an entry to CHANGELOG.md (description, pull request link, username(s))
• [ ] tried out on a mobile device (does everything work as expected on a smaller screen?)

[codecov[bot]]

Codecov Report

Merging #2677 (0484be9) into master (9673a1e) will increase coverage by 0.01%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2677      +/-   ##
==========================================
+ Coverage   75.63%   75.65%   +0.01%     
==========================================
  Files         386      386              
  Lines       41228    41228              
  Branches     2350     2352       +2     
==========================================
+ Hits        31182    31190       +8     
+ Misses      10046    10038       -8     

Files Changed	Coverage Δ
src/activities/components/ActivityItem.vue	92.82% <0.00%> (ø)

... and 1 file with indirect coverage changes

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