[Backgammon] Fix the number of legal_action_mask dimension.

[nissymori]

The number of dimension of legal_action_mask should be src * 6 = 156 but it was src * 6 + 6 = 162

• Fixed the dimension num of legal_action_mask and passed the test.
• Translated some remaining Japanese comments into English.

[codecov]

Codecov Report

Merging #1004 (d9beb64) into main (9e34323) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1004      +/-   ##
==========================================
+ Coverage   86.02%   86.06%   +0.03%     
==========================================
  Files          53       53              
  Lines        5897     5897              
==========================================
+ Hits         5073     5075       +2     
+ Misses        824      822       -2     

Files Changed	Coverage Δ
pgx/backgammon.py	99.07% <100.00%> (ø)

... and 1 file with indirect coverage changes