patch 9.0.0421: MS-Windows makefiles are inconsistently named

Problem:    MS-Windows makefiles are inconsistently named.
Solution:   Use consistent names. (Ken Takata, closes #11088)
diff --git a/src/testdir/README.txt b/src/testdir/README.txt
index b8bc52f..f72bdbf 100644
--- a/src/testdir/README.txt
+++ b/src/testdir/README.txt
@@ -106,7 +106,7 @@
 
 - To run the test on MS-Windows using the MSVC nmake:
 
-    > nmake -f Make_dos.mak
+    > nmake -f Make_mvc.mak
 
 - To run the tests with GUI Vim: