Fix Linkchecker Issues

[mmcky]

There are some broken links as reported by the nightly linkchecker run.

https://github.com/QuantEcon/lecture-python/runs/1690096183?check_suite_focus=true

@shlff would you be able to look through the log file under the linkcheck action and fix any items against broken that are displayed in the log.

writing output... [  4%] aiyagari
106

107
(line  241) ok        https://python-advanced.quantecon.org
108
(line  228) ok        https://github.com/QuantEcon/QuantEcon.py/blob/master/quantecon/markov/ddp.py
109
(line  228) redirect  http://quantecon.org/quantecon-py - with Found to https://quantecon.org/quantecon-py/
110
(line  241) ok        https://python-advanced.quantecon.org/discrete_dp.html
111
(line   73) broken    http://nbviewer.jupyter.org/github/QuantEcon/QuantEcon.notebooks/blob/master/aiyagari_continuous_time.ipynb - HTTPSConnectionPool(host='nbviewer.jupyter.org', port=443): Read timed out. (read timeout=30)
112
writing output... [  5%] ar1_processes

[shlff]

Thx @mmcky . I checked all links in the log file under the linkcheck action and find the following link is the only broken one detected there:

• https://nbviewer.jupyter.org/github/QuantEcon/QuantEcon.notebooks/blob/master/aiyagari_continuous_time.ipynb

A strange thing here is that this link seems NOT to be broken (please click the link above).

[mmcky]

thanks @shlff I will re-run the checker.

[mmcky]

thanks @shlff appears to be a false positive.