[stationary_densities] revert anchors to their respective targets

[najuzilu]

• ( line 305 ) https://python-intro.quantecon.org/finite_markov.html#Marginal-Distributions → https://python-intro.quantecon.org/finite_markov.html#mc_fdd
• ( line 348 ) https://python-intro.quantecon.org/finite_markov.html#Marginal-Distributions → https://python-intro.quantecon.org/finite_markov.html#mc_fddv
• ( line 671 ) https://python-intro.quantecon.org/finite_markov.html#Stationary-Distributions → https://python-intro.quantecon.org/finite_markov.html#mc_stat_dd