[css-background] Fix pluralization error

--HG--
extra : rebase_source : ab0cb14f129a81a0b93d8d8223644a9eafb1e8aa

Author: fantasai

css-backgrounds/Overview.html

@@ -3411,7 +3411,7 @@ <h3 id=the-border-image-width><span class=secno>6.3. </span>Drawing Areas:
      <th>Computed value:
 
      <td>four values, each a percentage, number, ‘<code
-      class=css>auto</code>’ keyword, or <var>&lt;length&gt;</var>s made
+      class=css>auto</code>’ keyword, or <var>&lt;length&gt;</var> made
       absolute
 
     <tr>
@@ -3521,7 +3521,7 @@ <h3 id=the-border-image-outset><span class=secno>6.4. </span>Edge Overhang:
     <tr>
      <th>Computed value:
 
-     <td>four values, each a number or <var>&lt;length&gt;</var>s made
+     <td>four values, each a number or <var>&lt;length&gt;</var> made
       absolute
 
     <tr>

css-backgrounds/Overview.src.html

@@ -2457,7 +2457,7 @@ <h3 id="the-border-image-width">Drawing Areas: the 'border-image-width' property
     <td>visual
   <tr>
     <th>Computed value:
-    <td>four values, each a percentage, number, ''auto'' keyword, or <var>&lt;length&gt;</var>s made absolute
+    <td>four values, each a percentage, number, ''auto'' keyword, or <var>&lt;length&gt;</var> made absolute
   <tr>
     <th>Animatable:
     <td>no
@@ -2534,7 +2534,7 @@ <h3 id="the-border-image-outset">Edge Overhang: the 'border-image-outset' proper
     <td>visual
   <tr>
     <th>Computed value:
-    <td>four values, each a number or <var>&lt;length&gt;</var>s made absolute
+    <td>four values, each a number or <var>&lt;length&gt;</var> made absolute
   <tr>
     <th>Animatable:
     <td>no