docs(mfbcf): 'storage coefficient' -> 'specific yield'

[wpbonelli]

Resolves #2268

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.3%. Comparing base (559ae74) to head (e5d4466).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2282     +/-   ##
=========================================
+ Coverage     73.9%   74.3%   +0.3%     
=========================================
  Files          294     294             
  Lines        59394   59392      -2     
=========================================
+ Hits         43932   44152    +220     
+ Misses       15462   15240    -222     

Files	Coverage Δ
flopy/mfusg/mfusgbcf.py	80.4% <ø> (ø)
flopy/modflow/mfbcf.py	89.1% <ø> (ø)

... and 15 files with indirect coverage changes