build: Correct explanation of unnest-vars example

Cc: Fam Zheng <fam@euphon.net>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20190213130240.15492-1-armbru@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

rules.mak

@@ -327,7 +327,7 @@ endef
 #     ../water/ice.mo-libs = -licemaker
 #     ../water/ice.mo-objs = ../water/ice1.o ../water/ice2.o
 #
-# Note that 'hot' didn't include 'season/' in the input, so 'summer.o' is not
+# Note that 'hot' didn't include 'water/' in the input, so 'steam.o' is not
 # included.
 #
 define unnest-vars