oauth(fix): remove double quotes from the default 'add to slack' img alt text

[zimeg]

Summary

This PR mirrors the changes contributed in slackapi/bolt-python#1170 🙏

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.66%. Comparing base (0861218) to head (b4c77a1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2044      +/-   ##
==========================================
- Coverage   92.51%   91.66%   -0.86%     
==========================================
  Files          37       37              
  Lines        9981     9981              
  Branches      631      631              
==========================================
- Hits         9234     9149      -85     
- Misses        747      820      +73     
- Partials        0       12      +12     

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	95.39% <ø> (-0.61%)	⬇️
oauth	77.39% <100.00%> (-4.13%)	⬇️
socket-mode	58.22% <ø> (-0.86%)	⬇️
web-api	96.89% <ø> (-0.40%)	⬇️
webhook	96.65% <ø> (-0.42%)	⬇️

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

[zimeg]

I wanted to double check once more that the HTML being updated is invalid and I can confirm this is true in at least Safari.

A double quoted alt text is the same as something like this (if I'm understanding the first equal to be the quoted string right):

- <img alt=""example"" />
+ <img alt="" />

Going to hold off on merging this right now, but I noticed a similar change will also be needed in Bolt for Java 👀

[zimeg]

The failing codecov check is also caused by a change to indirect coverage unrelated to this PR. However, I am open to including tests for this change if a specific check makes sense 🤔