Update ch-M2.ptx

fixed closed env

Author: fragandi

source/ch-M2.ptx

@@ -5,7 +5,7 @@
         <p>
 This chapter is co-authored by Francesca Gandini, Sumner Strom, Al Ashir Intisar
         </p>
- 
+  </introduction>
 
 
     <section xml:id="sec-m2-codespace">