doc: Fix build of German Cookbook translation.

This is a follow-up to f98e83a17f.
See <https://lists.gnu.org/archive/html/guix-devel/2020-03/msg00095.html>.
Reported there by Vagrant Cascadian <vagrant@debian.org>.

* po/doc/local.mk (DOC_COOKBOOK_PO_FILES): Use correct file name.
(doc-po-update-cookbook-%): Use correct PO file name.

po/doc/local.mk

@@ -24,7 +24,7 @@ DOC_PO_FILES=					\
   %D%/guix-manual.zh_CN.po
 
 DOC_COOKBOOK_PO_FILES=					\
-  %D%/guix-manual.de.po
+  %D%/guix-cookbook.de.po
 
 EXTRA_DIST = \
   %D%/guix-manual.pot \
@@ -58,7 +58,7 @@ doc-po-update-%:
 	fi
 
 doc-po-update-cookbook-%:
-	@lang=`echo "$@" | sed -e's/^doc-po-update-//'` ; \
+	@lang=`echo "$@" | sed -e's/^doc-po-update-cookbook-//'` ; \
 	output="$(srcdir)/po/doc/guix-cookbook.$$lang.po" ; \
 	input="$(srcdir)/po/doc/guix-cookbook.pot" ; \
 	if test -f "$$output"; then \