[css-writing-modes] Fix type <https://lists.w3.org/Archives/Public/www-style/2015Feb/0487.html>

Author: kojiishi

css-writing-modes/Overview.bs

@@ -2181,7 +2181,7 @@ Flow-Relative Mappings</h3>
     box and used to abstract layout rules related to the
     box properties (margins, borders, padding) and any properties
     related to positioning the box within its containing block
-    ('float', 'clear', 'top', 'bottom', 'left', 'right')
+    ('float', 'clear', 'top', 'bottom', 'left', 'right'.)
     For inline-level boxes, the writing mode of the <em>parent
     box</em> is used instead.