Added missing Oxford comma

P5/Source/Guidelines/en/SG-GentleIntroduction.xml

@@ -229,7 +229,7 @@ forth. And for certain types of analysis (most notably textual
 criticism) the physical appearance of one particular printed or
 manuscript source may be of importance:  paradoxically, one may wish to
 use descriptive markup to describe presentational features such as
-typeface, original topographic lineation, use of whitespace and so forth.</p>
+typeface, original topographic lineation, use of whitespace, and so forth.</p>
 
 <p>These textual structures overlap with one another in complex and
 unpredictable ways. Particularly when dealing with texts as