org: Change org-insert-structure-template to C-c C-,

* lisp/org.el (org-mode-map):
* doc/org-manual.org (With): Change keybinding of
  org-insert-structure-template.

See the thread titled "Poll: new keybinding for
org-insert-structure-template?" in December 2017 for details.

doc/org-manual.org

@@ -18170,7 +18170,7 @@ With just a few keystrokes, it is possible to insert empty structural
 blocks, such as =#+BEGIN_SRC= ... =#+END_SRC=, or to wrap existing
 text in such a block.
 
-- {{{kbd(C-c C-x w)}}} (~org-insert-structure-template~) ::
+- {{{kbd(C-c C-,)}}} (~org-insert-structure-template~) ::
 
      Prompt for a type of block structure, and insert the block at
      point.  If the region is active, it is wrapped in the block.

lisp/org.el

@@ -19157,8 +19157,7 @@ COMMANDS is a list of alternating OLDDEF NEWDEF command names."
 (org-defkey org-mode-map (kbd "C-c C-x E") #'org-inc-effort)
 (org-defkey org-mode-map (kbd "C-c C-x o") #'org-toggle-ordered-property)
 (org-defkey org-mode-map (kbd "C-c C-x i") #'org-columns-insert-dblock)
-(org-defkey org-mode-map (kbd "C-c C-x w") #'org-insert-structure-template)
-
+(org-defkey org-mode-map (kbd "C-c C-,") #'org-insert-structure-template)
 (org-defkey org-mode-map (kbd "C-c C-x .") #'org-timer)
 (org-defkey org-mode-map (kbd "C-c C-x -") #'org-timer-item)
 (org-defkey org-mode-map (kbd "C-c C-x 0") #'org-timer-start)