Correct grammar mistake

Change 'an two-byte' to 'a two-byte'

xml/System.IO/BinaryWriter.xml

@@ -1615,7 +1615,7 @@ This member is an explicit interface member implementation. It can be used only
       </Parameters>
       <Docs>
         <param name="value">The two-byte floating-point value to write.</param>
-        <summary>Writes an two-byte floating-point value to the current stream and advances the stream position by two bytes.</summary>
+        <summary>Writes a two-byte floating-point value to the current stream and advances the stream position by two bytes.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[