org.texi: Add `org-capture-templates' to variables index

* doc/org.texi (Capture templates): Add `org-capture-templates' to
  variables index.

doc/org.texi

@@ -7060,6 +7060,7 @@ through the customize interface.
 
 @table @kbd
 @orgkey{C-c c C}
+@vindex org-capture-templates
 Customize the variable @code{org-capture-templates}.
 @end table