Modify default tau value to 25/300

[vivekjoshy]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added test(s) covering the changes (if testable)

Change tau to 25/300

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under openskill.py's MIT license.

I certify the above statement is true and correct: @daegontaven

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (b194f51) compared to base (e6b194b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          565       573    +8     
  Branches       138       139    +1     
=========================================
+ Hits           565       573    +8     

Impacted Files	Coverage Δ
openskill/constants.py	100.00% <100.00%> (ø)
openskill/rate.py	100.00% <100.00%> (ø)

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