[gtk-doc] ci: fix pip3 syntax (wrong manpage)

[Stefan Sauer <stefkost src gnome org>]

commit ef5aa1bd771c9130b5388e1844a563591ef6c23e
Author: Stefan Sauer <ensonic users sf net>
Date:   Fri Dec 14 17:22:00 2018 +0100

    ci: fix pip3 syntax (wrong manpage)

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a242def..b805980 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -8,7 +8,7 @@ before_script:
                 libglib2.0-dev
                 python3 python3-dev python3-lxml python3-parameterized
                 python3-pip python3-pygments python3-unittest2
-  - pip3 --no-index install anytree
+  - pip3 install --no-index anytree
   - export LANG=C.UTF-8
 
 stages: