Minor issues in optimal stopping and stochastic discounting codebook

[HumphreyYang]

Optimal Stopping:

The Julia version of the strike plot

plot_strike(savefig=true)

failed to converge in the current build:

https://quantecon.github.io/book-dp1-public-companion/jl/optimal_stopping.html

Stochastic Discounting:

The interest rate plots in the Python version for the stochastic discounting chapter have too many x labels:

https://quantecon.github.io/book-dp1-public-companion/py/stochastic_discounting.html