FIX: 'View source' link

[AakashGfude]

The default branch is 'main'.

If any other branch is required then we need to set the option

html_theme_options:
     nb_branch: <branch_name>

fixes https://github.com/QuantEcon/quantecon-book-theme/issues/202

[codecov[bot]]

Codecov Report

Merging #203 (f494f07) into master (558393d) will decrease coverage by 0.61%. The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
- Coverage   71.72%   71.12%   -0.61%     
==========================================
  Files           2        2              
  Lines         237      239       +2     
==========================================
  Hits          170      170              
- Misses         67       69       +2     

Flag	Coverage Δ
pytests	71.12% <20.00%> (-0.61%)	:arrow_down:

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Impacted Files	Coverage Δ
quantecon_book_theme/__init__.py	60.38% <20.00%> (-0.80%)	:arrow_down:

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