FIX: conda -> pip for collab compat (#426)

* FIX: conda -> pip for collab compat

* fix linkchecker reported issue

Author: mmcky

lectures/troubleshooting.md

@@ -46,8 +46,8 @@ You also need to keep the external code libraries, such as [QuantEcon.py](https:
 
 For this task you can either
 
-* use conda install -y quantecon on the command line, or
-* execute !conda install -y quantecon within a Jupyter notebook.
+* use `pip install --upgrade quantecon` on the command line, or
+* execute `!pip install --upgrade quantecon` within a Jupyter notebook.
 
 If your local environment is still not working you can do two things.
 

lectures/wald_friedman.md

@@ -754,7 +754,7 @@ This leads to him having a higher expected loss when he puts equal weight on bot
 ### A Notebook Implementation
 
 To facilitate comparative statics, we provide
-a [Jupyter notebook](https://nbviewer.org/github/QuantEcon/lecture-python.notebooks/blob/master/wald_friedman.ipynb) that
+a [Jupyter notebook](https://nbviewer.org/github/QuantEcon/lecture-python.notebooks/blob/main/wald_friedman.ipynb) that
 generates the same plots, but with sliders.
 
 With these sliders, you can adjust parameters and immediately observe