Fix bugs in minimal Alphabet interface example

[jakobnissen]

Fix two small bugs in the documented interface example of BioSequence.

[codecov]

Codecov Report

Merging #208 (023f13b) into master (4cdc27e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #208   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files          31       31           
  Lines        2405     2405           
=======================================
  Hits         2122     2122           
  Misses        283      283           

Flag	Coverage Δ
unittests	88.23% <ø> (ø)

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