Fix X509Certificate.GetSerialNumberString to indicate correct endianness. (#11486)

Author: vcsjones

xml/System.Security.Cryptography.X509Certificates/X509Certificate.xml

@@ -3308,8 +3308,8 @@ The output of this method is equivalent to the output of the <xref:System.Securi
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>Returns the serial number of the X.509v3 certificate as a little-endian hexadecimal string .</summary>
-        <returns>The serial number of the X.509 certificate as a little-endian hexadecimal string.</returns>
+        <summary>Returns the serial number of the X.509v3 certificate as a big-endian hexadecimal string.</summary>
+        <returns>The serial number of the X.509 certificate as a big-endian hexadecimal string.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[