Add missing space (dotnet#1558)

Author: tkp1n

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

@@ -961,7 +961,7 @@
  ASN.1 DER is the only certificate format supported by this class.  
   
 > [!NOTE]
->  The certificate file is not restricted to .cerfiles. Any PKCS7 signed file can be used, including an Authenticode signed .pfx file.  
+>  The certificate file is not restricted to .cer files. Any PKCS7 signed file can be used, including an Authenticode signed .pfx file.  
   
    
   
@@ -3271,4 +3271,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>