[css-writing-modes] UTR50 was upgraded to UAX50, so update references accordingly. Fixes w3c#4160

Author: fantasai

css-writing-modes-4/Overview.bs

@@ -1408,10 +1408,10 @@ Vertical Typesetting and Font Features</h4>
         and East Asian fonts typically lack vertical substitutions for Western codepoints.)
         Unicode published draft data on which characters should appear sideways
         as the SVO property in <a href="http://www.unicode.org/reports/tr50/tr50-6.Orientation.txt">this data file</a>;
-        however, this property has been abandoned for the current revision of [[!UTR50]].
+        however, this property has been abandoned for the current revision of [[!UAX50]].
 
       <p><a>Typographic character units</a> which are classified
-      as <code>Tr</code> or <code>Tu</code> in [[!UTR50]]
+      as <code>Tr</code> or <code>Tu</code> in [[!UAX50]]
       are expected to have alternate glyphs or positioning for typesetting upright in vertical text.
       In the case of <code>Tr</code> <a>characters</a>,
       if such vertical alternate glyphs are missing from the font,
@@ -1436,7 +1436,7 @@ Vertical Typesetting and Font Features</h4>
 Mixed Vertical Orientations</h4>
 
   <p>
-    [[!UTR50]] defines the <code>Vertical_Orientation</code> property
+    [[!UAX50]] defines the <code>Vertical_Orientation</code> property
     for the default glyph orientation of mixed-orientation vertical text.
     When 'text-orientation' is ''mixed'',
     the UA must determine the orientation of each <a>typographic character unit</a>
@@ -1447,15 +1447,15 @@ Mixed Vertical Orientations</h4>
     if its orientation property is <code>R</code>.
 
   <p class="note">
-    Note that UTR50 does not handle scripts that rotate -90&deg; in vertical contexts,
+    Note that UAX50 does not handle scripts that rotate -90&deg; in vertical contexts,
     so they will not be typeset correctly with ''mixed'' orientation.
     Use ''sideways-lr'' for such scripts.
 
   <p class="note">
     The OpenType ''vrt2'' feature, which is intended for mixed-orientation typesetting,
     is not used by CSS.
     It delegates the responsibility for orienting glyphs to the font designer.
-    CSS instead dictates the orientation through [[!UTR50]]
+    CSS instead dictates the orientation through [[!UAX50]]
     and orients glyphs by typesetting them sideways or upright as appropriate.
 
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