Replace centering by an environment

More LaTeX idomatic

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

@@ -107,13 +107,14 @@ starter's flavor by changing the type to a liquid starter.
 \label{section:liquid-starter}
 
 \begin{figure}[!htb]
-  \centering
+\begin{center}
   \includegraphics[width=0.5\textwidth]{sourdough-starter-liquid.jpg}
   \caption{A liquid sourdough starter features a high level of water. The high
   water amount boosts lactic acid producing bacteria. After a while the liquid
   and flour start to separate. Bubbles on the side of the flour
   indicate that the starter is ready to be used.}%
   \label{fig:liquid-sourdough-starter}
+\end{center}
 \end{figure}