Documentation/sphinx: include admin-guide in the latex/pdf build

Fix the warning:

WARNING: "latex_documents" config value references unknown document
user/index
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Documentation/conf.py

@@ -336,7 +336,7 @@ latex_elements = {
 # (source start file, target name, title,
 #  author, documentclass [howto, manual, or own class]).
 latex_documents = [
-    ('user/index', 'linux-user.tex', 'Linux Kernel User Documentation',
+    ('admin-guide/index', 'linux-user.tex', 'Linux Kernel User Documentation',
      'The kernel development community', 'manual'),
     ('kernel-documentation', 'kernel-documentation.tex', 'The Linux Kernel Documentation',
      'The kernel development community', 'manual'),