Update SHA512.xml (dotnet#9119)

Typo fixed SHA51 replaced by SHA512

Author: wollmich

xml/System.Security.Cryptography/SHA512.xml

@@ -258,7 +258,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- Possible values for `hashName` are SHA512, System.Security.Cryptography.SHA51, System.Security.Cryptography.SHA512Managed, and System.Security.Cryptography.SHA512CryptoServiceProvider.  
+ Possible values for `hashName` are SHA512, System.Security.Cryptography.SHA512, System.Security.Cryptography.SHA512Managed, and System.Security.Cryptography.SHA512CryptoServiceProvider.  
   
  ]]></format>
         </remarks>