Fix a typo in ex_4_walking (#209)

* [exercise] fix typo in ex_4_walking

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: Pierre-Alexandre Leziart <paleziart@laas.fr>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

exercizes/ex_4_walking.py

@@ -180,7 +180,7 @@ for i in range(-N_pre, N + N_post):
                 cop_RF[0, i] = f_RF[4, i] / f_RF[2, i]
                 cop_RF[1, i] = -f_RF[3, i] / f_RF[2, i]
         if tsid_biped.formulation.checkContact(tsid_biped.contactLF.name, sol):
-            T_LF = tsid_biped.contactRF.getForceGeneratorMatrix
+            T_LF = tsid_biped.contactLF.getForceGeneratorMatrix
             f_LF[:, i] = T_LF.dot(
                 tsid_biped.formulation.getContactForce(tsid_biped.contactLF.name, sol)
             )