Makefile: Fix help text for target 'installer'

The NSIS based installer currently does not install qemu-ga.
It installs the executables and other files for the QEMU system emulation.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

Makefile

@@ -695,7 +695,7 @@ help:
 	@echo  ''
 ifdef CONFIG_WIN32
 	@echo  'Windows targets:'
-	@echo  '  installer       - Build NSIS-based installer for qemu-ga'
+	@echo  '  installer       - Build NSIS-based installer for QEMU'
 ifdef QEMU_GA_MSI_ENABLED
 	@echo  '  msi             - Build MSI-based installer for qemu-ga'
 endif