Improve the look of tables (#99)

In order tho make the tables look better in document:
    1) Remove the vertical separation
    2) Add different width for top/mid/bottom rules
    3) Center them in the page

Co-authored-by: Cédric Andrieu <cedric@cirrus.com>

book/sourdough-starter/sourdough-starter.tex

@@ -35,8 +35,10 @@ comes in handy. Let's look at the default recipe with baker's
 math and then adjust it for the 1.4 kilogram flour quantity.
 
 \begin{figure}[!htb]
+\begin{center}
   \includegraphics{tables/table-bakers-math-example.pdf}
   \caption{An example table demonstrating how to properly calculate using baker's math}
+\end{center}
 \end{figure}
 
 Note how each of the ingredients is calculated as a percentage
@@ -49,9 +51,11 @@ more flour available the next day. As mentioned the next day
 we have 1.4 kilograms at hand (1400 grams).
 
 \begin{figure}[H]
-  \includegraphics{tables/table-recipe-bakers-math.pdf}
-  \caption{An example recipe that uses 1400 grams as its baseline and
-  is then calculated using baker's math}
+    \begin{center}
+        \includegraphics{tables/table-recipe-bakers-math.pdf}
+        \caption{An example recipe that uses 1400 grams as its baseline and
+        is then calculated using baker's math}
+    \end{center}
 \end{figure}
 
 For each ingredient we calculate the percentage