patch 8.0.1197: MS-Windows build instructions are not up to date

Problem:    MS-Windows build instructions are not up to date.
Solution:   Adjust the instructions.  Fix the nsis script.
diff --git a/nsis/gvim.nsi b/nsis/gvim.nsi
index 1c548b3..8447d5f 100644
--- a/nsis/gvim.nsi
+++ b/nsis/gvim.nsi
@@ -351,7 +351,7 @@
 	  # Can't copy gvimext.dll, create it under another name and rename it
 	  # on next reboot.
 	  GetTempFileName $3 $0\GvimExt64
-	  File /oname=$3 ${VIMSRC}\GvimExt64\gvimext.dll
+	  File /oname=$3 ${VIMSRC}\GvimExt\gvimext64.dll
 	  Rename /REBOOTOK $3 $0\GvimExt64\gvimext.dll
 !ifdef HAVE_NLS
 	  GetTempFileName $3 $0\GvimExt64