fixed the default for IsTextSearchEnabled to true (dotnet#573)

Author: aramkoukia

xml/System.Windows.Controls/ItemsControl.xml

@@ -1071,7 +1071,7 @@ ListBox that contains multiple types of objects
  <Docs>
  <summary>Gets or sets a value that indicates whether <see cref="T:System.Windows.Controls.TextSearch" /> is enabled on the <see cref="T:System.Windows.Controls.ItemsControl" /> instance.</summary>
  <value>
- <see langword="true" /> if <see cref="T:System.Windows.Controls.TextSearch" /> is enabled; otherwise, <see langword="false" />. The default is <see langword="false" />.</value>
+ <see langword="true" /> if <see cref="T:System.Windows.Controls.TextSearch" /> is enabled; otherwise, <see langword="true" />. The default is <see langword="false" />.</value>
  <remarks>
  <format type="text/markdown"><![CDATA[ 
  
@@ -2765,4 +2765,4 @@ ListBox that contains multiple types of objects
  </Docs>
  </Member>
  </Members>
-</Type>
+</Type>