Rename Index.atIndex -> Index.at

[muukii]

I've renamed following.
PageIndex.atIndex(index: Int) -> PageIndex.at(index: Int)

Then you can initialize it as follows.

.at(index: 3)

What do yo think?

[codecov]

Codecov Report

Merging #42 into master will decrease coverage by 0.15%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   79.72%   79.56%   -0.16%     
==========================================
  Files          14       14              
  Lines         868      871       +3     
==========================================
+ Hits          692      693       +1     
- Misses        176      178       +2

Impacted Files	Coverage Δ
Sources/Pageboy/PageboyViewController.swift	77.14% <50%> (-0.77%)	⬇️

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[msaps]

Thanks, I like it! 👍

[muukii]

@msaps
Thank you for the merge!