[gexiv2] ci: Use proper command for enabling doc build

[Jens Georg <jensgeorg src gnome org>]

commit fc2004d0bbd3c2fa6c3bcf60e82fe75e420a3e3a
Author: Jens Georg <mail jensge org>
Date:   Thu May 24 07:03:30 2018 +0000

    ci: Use proper command for enabling doc build

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 283e79b..64edd44 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -51,7 +51,7 @@ pages:
   only:
     - master
   script:
-    - meson -Ddocumentation=true _build .
+    - meson -Denable-gtk-doc=true _build .
     - ninja -C _build test gexiv2-doc
     - mkdir -p public/
     - mv _build/docs/reference/html/ public/docs/