[Lesson 1] Fix bug in markdowns about macro for time-increment.
[Lesson 1] Fix bug in markdowns about macro for time-increment.

A macro was used to write the time-increment in markdown cells.
Now $\Delta t$ replaces $\dt$.

MohammedAttya/numerical-mooc,setiabudidaya/numerical-mooc,bbell59101/numerical-mooc,alexlib/numeric-mooc,morepj/numerical-mooc