Fix casing on the file to load in LoadCertificateFromFile

It is "MS.cer" in the testdata package.  This passed on Windows non-POSIX filesystems, but caused a failure for the wrong reason when trying to use the tests for TDD on Unix.

Author: bartonjs

src/System.Security.Cryptography.X509Certificates/tests/LoadFromFileTests.cs

@@ -152,7 +152,7 @@ public static void TestLoadSignedFile()
 
         private static X509Certificate2 LoadCertificateFromFile()
         {
-            string path = Path.Combine("TestData", "Ms.cer");
+            string path = Path.Combine("TestData", "MS.cer");
             if (!File.Exists(path))
                 throw new Exception(string.Format("Test infrastructure failure: Expected to find file \"{0}\".", path));
             byte[] data = File.ReadAllBytes(path);