Bump version number in portable installer to 3.2.0

packages/win_installer/portable/portable-comment.txt

 Path=.\Aegisub portable
-Title=Aegisub 3.1.0 Portable
+Title=Aegisub 3.2.0 Portable
 Text
 {
-<h1 style="font-family: Tahoma; font-size: 20px; text-align: center;">Aegisub 3.1.0 Portable</h1>
+<h1 style="font-family: Tahoma; font-size: 20px; text-align: center;">Aegisub 3.2.0 Portable</h1>
 <p style="font-family: Tahoma; font-size: 12px;">This archive contains a
 "portable" version of Aegisub, suitable for running from USB pen drives or on
 machines which you cannot install software on. To reduce the size several