Add troubleshooting: Too sour starter

I was asked on YouTube regarding this. I figured I add another
chapter on the topic in the troubleshooting section.

book/sourdough-starter/sourdough-starter-types.tex

@@ -145,6 +145,7 @@ drain the liquid part on your starter and use it. I have used it numerous
 times to make lactofermented hot sauces.
 
 \section{Stiff starter}
+\label{section:stiff-starter}
 
 \begin{figure}[!htb]
   \includegraphics[width=\textwidth]{sourdough-starter-stiff.jpg}