vpl: fix reference in comment to non-existing SPL_SERIAL_SUPPORT

Since commit 2a7360666871 ("serial: Rename SERIAL_SUPPORT to SERIAL")
SPL_SERIAL_SUPPORT is named SPL_SERIAL. So let's update the comment to
point to the correct Kconfig option in the comment of VPL_SERIAL.

Fixes: 747093dd408 ("vpl: Add Kconfig options for VPL")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/spl/Kconfig.vpl b/common/spl/Kconfig.vpl
index ba4b2e4..f331622 100644
--- a/common/spl/Kconfig.vpl
+++ b/common/spl/Kconfig.vpl
@@ -166,7 +166,7 @@
 	select VPL_PRINTF
 	select VPL_STRTO
 	help
-	  Enable support for serial in VPL. See SPL_SERIAL_SUPPORT for
+	  Enable support for serial in VPL. See SPL_SERIAL for
 	  details.
 
 config VPL_SIZE_LIMIT