Update NOTES.WIN

Make the recommendation for MSYS perl in an MSYS environment more forceful.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>

NOTES.WIN

diff --git a/NOTES.WIN b/NOTES.WIN
index b3d1967..00ffb4a 100644
--- a/NOTES.WIN
+++ b/NOTES.WIN
@@ -95,7 +95,7 @@
    required. Run the installers and do whatever magic they say it takes
    to start MSYS bash shell with GNU tools and matching Perl on its PATH.
    "Matching Perl" refers to chosen "shell environment", i.e. if built
-   under MSYS, then Perl compiled for MSYS is highly recommended.
+   under MSYS, then Perl compiled for MSYS must be used.
 
    Alternativelly, one can use MSYS2 from http://msys2.github.io/,
    which includes MingW (32-bit and 64-bit).