Configurations: fix typo in 50-masm.conf.

Reviewed-by: NRichard Levitte <levitte@openssl.org>

Configurations/50-masm.conf

@@ -12,6 +12,6 @@
         inherit_from    => [ "VC-WIN64A" ],
         as              => "ml64",
         asflags         => "/c /Cp /Cx /Zi",
-        asoutflags      => "/Fo",
+        asoutflag       => "/Fo",
     },
 );