manual generation: rename manual-txt into manual-text

The output extension and the generation messages refer to 'text', but the make
target was confusingly 'txt'. This patch changes the make target for
consistency.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/Makefile b/Makefile
index f266e2d..1496bd7 100644
--- a/Makefile
+++ b/Makefile
@@ -797,7 +797,7 @@
 	@echo '  manual-html            - build manual in HTML'
 	@echo '  manual-split-html      - build manual in split HTML'
 	@echo '  manual-pdf             - build manual in PDF'
-	@echo '  manual-txt             - build manual in txt'
+	@echo '  manual-text            - build manual in text'
 	@echo '  manual-epub            - build manual in ePub'
 	@echo
 	@echo 'Miscellaneous:'
@@ -821,7 +821,7 @@
 # documentation to the git output
 release:
 	git archive --format=tar --prefix=$(OUT)/ HEAD > $(OUT).tar
-	$(MAKE) O=$(OUT) manual-html manual-txt manual-pdf
+	$(MAKE) O=$(OUT) manual-html manual-text manual-pdf
 	tar rf $(OUT).tar $(OUT)
 	gzip -9 -c < $(OUT).tar > $(OUT).tar.gz
 	bzip2 -9 -c < $(OUT).tar > $(OUT).tar.bz2