doc: update help message

Update the help message used for 'make help':

  Documentation targets:
    Linux kernel internal documentation in different formats from ReST:
=>
    U-Boot documentation in different formats from ReST:

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/Makefile b/doc/Makefile
index 0e0da56..a686d47 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -106,7 +106,7 @@
 	$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=doc/media clean
 
 dochelp:
-	@echo  ' Linux kernel internal documentation in different formats from ReST:'
+	@echo  ' U-Boot documentation in different formats from ReST:'
 	@echo  '  htmldocs        - HTML'
 	@echo  '  latexdocs       - LaTeX'
 	@echo  '  pdfdocs         - PDF'