Improve heading and sentence

wiki/src/contribute/how/documentation/style_guide.mdwn

@@ -5,10 +5,10 @@
   Refer to this article from NN/g on [presenting bulleted
   lists](https://www.nngroup.com/articles/presenting-bulleted-lists/).
 
-- **Debian and Ubuntu version**
+- **Debian and Ubuntu versions**
 
-  Refer to Debian versions primarily by their number, and additionally
-  by their codename.
+  Refer to Debian and Ubuntu versions primarily by their numbers, and additionally
+  by their codenames.
 
   * *For example*:
     - Tails 3.0 is based on Debian 9 (Stretch)