rename var mu to learning_rate

New var mu defined as Learning rate, but later in the same code cell var
learning_rate is used instead, wich was defined in previous cell.
Suggest to rename becouse cell uses var which is not defined in it, but
defines var wich is not used.
Suggest to rename mu to learning_rate (not learning_rate to mu later in
code) because this code cell is supposed to be like previous but with
comments. And in previous cell the name learning_rate was used.
Commit also change var name in comment to reflect changes in var
definition.