kbuild: fix a typo dt_binding_schema -> dt_binding_schemas

If we follow "make help" to "make dt_binding_schema", we will see
below error:

$ make dt_binding_schema
make[1]: *** No rule to make target 'dt_binding_schema'.  Stop.
make: *** [Makefile:224: __sub-make] Error 2

It should be a typo. So this will fix it.

Fixes: 604a57ba ("dt-bindings: kbuild: Add separate target/dependency for processed-schema.json")
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1645,7 +1645,7 @@ help:
 		echo '* dtbs               - Build device tree blobs for enabled boards'; \
 		echo '  dtbs_install       - Install dtbs to $(INSTALL_DTBS_PATH)'; \
 		echo '  dt_binding_check   - Validate device tree binding documents and examples'; \
-		echo '  dt_binding_schema  - Build processed device tree binding schemas'; \
+		echo '  dt_binding_schemas - Build processed device tree binding schemas'; \
 		echo '  dtbs_check         - Validate device tree source files';\
 		echo '')