Fix missing s per @sydb.

Author: martindholmes

P5/Source/Guidelines/en/WD-NonStandardCharacters.xml

@@ -1246,7 +1246,7 @@ precinct at Dodona. (L.H. Jeffery Archive)</head>
  <lb/>ΑΣΣΑΙ
  </ab></egXML>
   <p>The 180-degree rotation around the Y (vertical) axis here
- describes what is happening in the RTL  line in boustrophedon; the order of glyphs
+ describes what is happening in the RTL lines in boustrophedon; the order of glyphs
  is reversed, and so is their individual orientation (in fact, we see them
  <soCalled>from the back</soCalled>, as it were). <gi>seg</gi> elements
  have been used here because these are clearly not <soCalled>lines</soCalled>