adds p element to introduction elements (#189)

**Pull Request Description**

Ensures that all `introduction`, `statement` and `hint` elements use
`<p>` elements.

Closes #156 

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Author: braughtg

source/ch-communities-and-collaboration/sec-basic-foss-workflow.ptx

@@ -30,7 +30,9 @@
     <title />
 
     <introduction>
-      The exercises in this section will help to familiarize you with the terminology and the process of this basic FOSS workflow.
+      <p>
+        The exercises in this section will help to familiarize you with the terminology and the process of this basic FOSS workflow.
+      </p>
     </introduction>
 
     <exercise 

source/ch-instructor-guide/sec-instructor-intro.ptx

@@ -627,6 +627,7 @@
 		<title>Instructor Materials</title>
 
 		<introduction>
+			<p>
 			The following links provide easy access to the chapters with the hands-on exercises, the presentation slides for each chapter and a set of instructor notes for each chapter. 
 
 			The <term>Instructor Notes</term> for each chapter include <term>Slide Notes</term>, a <term>To-Do List</term>, and <term>Exercise Notes</term>. The Slide Notes give an overview of the slides and then slide-by-slide information for the instructor. The To-Do List enumerates any tasks that the instructor should be sure to do before the class meeting.  The Exercise Notes provide some helpful comments on specific hands-on exercises.
@@ -696,6 +697,7 @@
 					</p>
 				</li>
 			</ul>	
+			</p>
 		</introduction>
 	</subsection>