Fixed file naming in build script

There was a file naming issue (ZMemoryx.y.z.pk3 instead of ZMemory_vx.y.z.pk3)
This has been fixed.
The file on the release has been updated as well but the commit pointed to the tag hasn't been altered. It isn't important at this stage anyway.

build.sh

@@ -14,7 +14,7 @@ version=$(cat ../version.txt)
 if [[ $thin_arg == "thin" ]]; then
   7za a -tzip ../ZMemory_v$version\_thin.pk3 -r -x\!*.sh -x\!acs/.gitignore -xr\!acs_source
 else
-  7za a -tzip ../ZMemory$version.pk3 -r -x\!*.sh -x\!acs/.gitignore
+  7za a -tzip ../ZMemory_v$version.pk3 -r -x\!*.sh -x\!acs/.gitignore
 fi