Change "\" and "~" syntax to symbol

* lisp/org.el (org-mode): Change "\" and "~" characters syntax from
  `punctuation' to `symbol' so they are on par with other characters used
  in Org syntax (e.g., "/", "*"...).

This change is needed to correctly find radio links starting with an
entity:

  <<<\alpha-test>>> \alpha-test

lisp/org.el

@@ -5341,6 +5341,8 @@ The following commands are available:
   ;; Modify a few syntax entries
   (modify-syntax-entry ?@ "w")
   (modify-syntax-entry ?\" "\"")
+  (modify-syntax-entry ?\\ "_")
+  (modify-syntax-entry ?~ "_")
   (if org-startup-truncated (setq truncate-lines t))
   (when org-startup-indented (require 'org-indent) (org-indent-mode 1))
   (org-set-local 'font-lock-unfontify-region-function