docs: switch ObservedObject to StateObject

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

The docs and Playgrounds currently use @ObservedObject for SwiftUI examples where @StateObject is more suitable.

Related issue: #n/a

Approach

Update documentation

TODOs before merging

• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #343 (efe1abd) into main (69dce16) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
+ Coverage   85.16%   85.18%   +0.01%     
==========================================
  Files         114      114              
  Lines       12198    12198              
==========================================
+ Hits        10389    10391       +2     
+ Misses       1809     1807       -2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseObject.swift	82.29% <0.00%> (+0.30%)	⬆️

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