Update String.xml (#11621)

Author: PesBandi

xml/System/String.xml

@@ -6261,7 +6261,7 @@ The index of a format item is less than zero, or greater than two.</exception>
         <param name="arg0">The first object to format.</param>
         <param name="arg1">The second object to format.</param>
         <param name="arg2">The third object to format.</param>
-        <summary>Replaces the format items in a string with the string representation of three specified objects. An parameter supplies culture-specific formatting information.</summary>
+        <summary>Replaces the format items in a string with the string representation of three specified objects. A parameter supplies culture-specific formatting information.</summary>
         <returns>A copy of <paramref name="format" /> in which the format items have been replaced by the string representations of <paramref name="arg0" />, <paramref name="arg1" />, and <paramref name="arg2" />.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[