Use 1/24em as a fallback for spaceAfterScript

https://github.com/mathml-refresh/mathml/issues/69

Author: fred-wang

index.html

@@ -5173,7 +5173,7 @@ <h3>Layout constants (<code>MathConstants</code>)</h3>
           <dt><dfn>superscriptBottomMaxWithSubscript</dfn></dt>
           <dd><code>MATH.MathConstants.superscriptBottomMaxWithSubscript</code> or ⅘ × <code>OS/2.sxHeight</code> if the constant is not available.</dd>
           <dt><dfn>spaceAfterScript</dfn></dt>
-          <dd><code>MATH.MathConstants.spaceAfterScript</code> or ?? if the constant is not available.</dd>
+          <dd><code>MATH.MathConstants.spaceAfterScript</code> or 1/24em if the constant is not available.</dd>
           <dt><dfn>upperLimitGapMin</dfn></dt>
           <dd><code>MATH.MathConstants.upperLimitGapMin</code> or ?? if the constant is not available.</dd>
           <dt><dfn>upperLimitBaselineRiseMin</dfn></dt>