fixed small typo in matrix_input.py
fixed small typo in matrix_input.py

mwest1066/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,jakebailey/PrairieLearn,mwest1066/PrairieLearn,jakebailey/PrairieLearn,jakebailey/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,jakebailey/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,tbretl/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,tbretl/PrairieLearn,parasgithub/PrairieLearn