Fix a few docstrings.

Thanks to Paul Eggert for fixing this in Emacs.

lisp/org.el

@@ -2651,7 +2651,7 @@ To turn this on on a per-file basis, insert anywhere in the file:
 (defcustom org-time-stamp-custom-formats
   '("<%m/%d/%y %a>" . "<%m/%d/%y %a %H:%M>") ; american
   "Custom formats for time stamps.  See `format-time-string' for the syntax.
-These are overlayed over the default ISO format if the variable
+These are overlaid over the default ISO format if the variable
 `org-display-custom-times' is set.  Time like %H:%M should be at the
 end of the second format.  The custom formats are also honored by export
 commands, if custom time display is turned on at the time of export."
@@ -15349,7 +15349,7 @@ The command returns the inserted time stamp."
   (org-restart-font-lock)
   (setq org-table-may-need-update t)
   (if org-display-custom-times
-      (message "Time stamps are overlayed with custom format")
+      (message "Time stamps are overlaid with custom format")
     (message "Time stamp overlays removed")))
 
 (defun org-display-custom-time (beg end)
@@ -19436,7 +19436,7 @@ contexts are:
 :target           on a <<target>>
 :radio-target     on a <<<radio-target>>>
 :latex-fragment   on a LaTeX fragment
-:latex-preview    on a LaTeX fragment with overlayed preview image
+:latex-preview    on a LaTeX fragment with overlaid preview image
 
 This function expects the position to be visible because it uses font-lock
 faces as a help to recognize the following contexts: :table-special, :link,