Fix another build error for help/reference/shell/meson.build

It fixes error

help/reference/shell/meson.build:16:6: ERROR: gnome.gtkdoc got unknown keyword arguments "cflags"

Author: timp87

help/reference/shell/meson.build

@@ -24,7 +24,7 @@ gnome.gtkdoc(
     ignore_headers: [libview_private_headers],
     dependencies: xreader_doc_deps,
     mkdb_args: ['--xml-mode', '--output-format=xml', '--name-space=ev'],
-    cflags: '-DXREADER_COMPILATION',
+    c_args: '-DXREADER_COMPILATION',
     fixxref_args: fixref_args,
     install: true,
     install_dir: meson.project_name(),