patch 8.1.0712: MS-Windows build instructions are a bit outdated

Problem:    MS-Windows build instructions are a bit outdated.
Solution:   Update the instructions. (Ken Takata)
diff --git a/src/version.c b/src/version.c
index 387b7b8..aaf12c2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    712,
+/**/
     711,
 /**/
     710,