Small bug + fix for VS2008 builds of JuceDemoPlugin

[with latest pull from git]

Building the JuceDemoPlugin project with VS2008 gives an error in the post-build step:

copy /Y "$(OutDir)$(TargetFileName)" "$(OutDir)$(TargetName).vst3"

gives:

copy /Y ".\DebugJuceDemoPlugin.dll" ".\DebugJuceDemoPlugin.vst3"

Fix: it needs two extra slashes:

copy /Y "$(OutDir)\$(TargetFileName)" "$(OutDir)\$(TargetName).vst3"

(I didn't check the VS2005 build)

Thanks, will attend to that..