fix: contact force legend

[joshuafairch]

Flipped master/slave in contact plot legend

[joshuafairch]

Verified plt.plot(t_vals, FX.get_field({"interface": 19, "idtype": 0}).data, label="FX, slave") plots positive contact force. This is originally noted as the slave side.
Verified LSPP binout_matsum, contact 19, slave, fx plots negative contact force.

Reversed slave/master notation in legends accordingly.

Preceding paragraph description "(0 for the master side of the interface, 1 for the slave)" is correct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.90%. Comparing base (3deb5ec) to head (830c56b).
Report is 10 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2280      +/-   ##
==========================================
+ Coverage   83.78%   83.90%   +0.12%     
==========================================
  Files          90       90              
  Lines       10396    10437      +41     
==========================================
+ Hits         8710     8757      +47     
+ Misses       1686     1680       -6