Fix duplicate entry in history

Thanks @OzTK for the fixes.

book/history/sourdough-history.tex

@@ -21,17 +21,6 @@ \chapter{The history of sourdough}%
 the evening of December~31.  It seems that humans managed to arrive just
 in time to join the celebration at the end of the year.
 
-The story of sourdough bread begins in ancient oceans. These oceans were the
-birthplace of all Earth's life. To better envision the vast history of
-our planet lets create a timeline of 1~year. On this scale, January~1 signifies Earth's
-formation 4.54~billion years ago. Midnight on December~31 is our present.
-Each day represents roughly 12~million years. This technique simplifies the
-complexity of time but also renders the extraordinary expanse of our planet's
-history into a more graspable frame. We humans are in fact a recent addition
-to our planet, so young that we made appearance on the evening of December~31.
-It seems that humans managed to arrive just in time to join
-the celebration at year's end.
-
 On March~25, the oceans birthed the first single-celled bacteria. In these
 waters, another single-celled life form, \emph{archaea}, also thrived. These
 organisms inhabit extreme environments, from boiling vents to icy waters.
@@ -47,7 +36,7 @@ \chapter{The history of sourdough}%
 \end{center}
 \end{figure}
 
-Whoever comes first first, bacteria or archaea, remains debated. For three
+Whoever comes first, bacteria or archaea, remains debated. For three
 months (or approximately 1.1~billion years), these life forms dominated
 the oceans. Then, on June~25 in an highly unlikely event, an archaeon consumed a bacterium.
 Instead of digesting it, they formed a symbiotic relationship. This led to the