DEVELOPERS: fix docs/manual/adding-packages-meson.txt reference

Commit 32cec3be976 (docs/manual: rename *.txt as *.adoc) renamed the manual
files but forgot to update the reference in the DEVELOPERS file, causing
check-package to warn:

WARNING: 'docs/manual/adding-packages-meson.txt' doesn't match any file, line 851

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

DEVELOPERS

@@ -848,7 +848,7 @@ F:	package/bayer2rgb-neon/
 F:	package/gstreamer1/gst1-plugins-bayer2rgb-neon/
 
 N:	Eric Le Bihan <eric.le.bihan.dev@free.fr>
-F:	docs/manual/adding-packages-meson.txt
+F:	docs/manual/adding-packages-meson.adoc
 F:	package/adwaita-icon-theme/
 F:	package/darkhttpd/
 F:	package/eudev/