Bitfield::with_capacity fix OutOfBounds i field, plus doc typo fix

[carver]

The OutOfBounds error was not filling the i field correctly, and the docs mistakenly said the method returns None instead of Err if the index is too large.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.76%. Comparing base (dc04526) to head (90ed130).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ssz/src/bitfield.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   77.48%   77.76%   +0.27%     
==========================================
  Files          12       12              
  Lines         715      715              
==========================================
+ Hits          554      556       +2     
+ Misses        161      159       -2     

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[michaelsproul]

Nice catch, thank you!