Fix typo in history (#391)

book/history/sourdough-history.tex

@@ -38,7 +38,7 @@ \chapter{The history of sourdough}%
 
 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.
+the oceans. Then, on June~25 in a highly unlikely event, an archaeon consumed a bacterium.
 Instead of digesting it, they formed a symbiotic relationship. This led to the
 first nucleated organisms, marking an evolutionary milestone. This event lead
 to the development of plants, fungi and also ultimately humans.