Add proxyUrl in api client config document

[seratch]

Summary

This pull request adds proxyUrl part in SlackConfig document page - https://slack.dev/java-slack-sdk/guides/web-api-basics#customize-your-slack-api-clients

🇯🇵

Requirements (place an x in each [ ])

• 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

Merging #497 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #497      +/-   ##
============================================
- Coverage     83.93%   83.87%   -0.07%     
+ Complexity     2416     2415       -1     
============================================
  Files           252      252              
  Lines          6492     6492              
  Branches        574      574              
============================================
- Hits           5449     5445       -4     
- Misses          687      691       +4     
  Partials        356      356              

Impacted Files	Coverage Δ	Complexity Δ
...pi/methods/metrics/impl/RedisMetricsDatastore.java	87.03% <0.00%> (-2.47%)	37.00% <0.00%> (-1.00%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 425bb2b...9cb3575. Read the comment docs.