Updated pip command for repository install

[jaymedina]

Updated the pip command for installing the bleeding edge version of astroquery. The command in place was returning an error due to pip not finding astropy_helpers even after manually installing it.

[codecov]

Codecov Report

Merging #2675 (01e0474) into main (5e221bc) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 01e0474 differs from pull request most recent head 8fc3440. Consider uploading reports for the commit 8fc3440 to get more accurate results

@@            Coverage Diff             @@
##             main    #2675      +/-   ##
==========================================
+ Coverage   69.16%   69.19%   +0.02%     
==========================================
  Files         304      304              
  Lines       22539    22539              
==========================================
+ Hits        15589    15595       +6     
+ Misses       6950     6944       -6     

Impacted Files	Coverage Δ
astroquery/mast/cutouts.py	66.54% <0.00%> (+2.15%)	⬆️

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[ceb8]

Looks good to me.

[bsipocz]

While we're at it, I'm pushing another commit to update the source install instructions, too.