Correct installation directory of GVariant specification

548ec9f186d373c4e55528a04d78450f6f2fceef accidentally moved the GVariant
spec to the toplevel /usr/share/doc directory, which is surely not
right. Let's move it back into the glib-2.0 subdirectory.

It's debatable whether this is the best place to install the GVariant
specification, since it's not part of the gi-docgen docs, but surely
it's much better than not putting it in any subdirectory.

Fixes #3351

docs/reference/glib/meson.build

diff --git a/docs/reference/glib/meson.build b/docs/reference/glib/meson.build
index e5e6726..47c5ecf 100644
--- a/docs/reference/glib/meson.build
+++ b/docs/reference/glib/meson.build
@@ -23,7 +23,7 @@
   rst2html5 = find_program('rst2html5', 'rst2html5.py', required: false)
 
   if rst2html5.found()
-    spec_path = docs_dir
+    spec_path = docs_dir / 'glib-2.0'
 
     figures = files(
       'gvariant-byte-boundaries.svg',