Fix typos (already fixed in Emacs trunk.)

Thanks to Juanma Barranquero for spotting and fixing them in Emacs.

lisp/ob-ref.el

@@ -73,7 +73,7 @@
   "Parse a variable ASSIGNMENT in a header argument.
 If the right hand side of the assignment has a literal value
 return that value, otherwise interpret as a reference to an
-external resource and find it's value using
+external resource and find its value using
 `org-babel-ref-resolve'.  Return a list with two elements.  The
 first element of the list will be the name of the variable, and
 the second will be an emacs-lisp representation of the value of

lisp/ob.el

@@ -540,7 +540,7 @@ block."
 
 (defun org-babel-expand-body:generic (body params &optional var-lines)
   "Expand BODY with PARAMS.
-Expand a block of code with org-babel according to it's header
+Expand a block of code with org-babel according to its header
 arguments.  This generic implementation of body expansion is
 called for languages which have not defined their own specific
 org-babel-expand-body:lang function."
@@ -2217,7 +2217,7 @@ appropriate."
     cell))
 
 (defun org-babel-number-p (string)
-  "If STRING represents a number return it's value."
+  "If STRING represents a number return its value."
   (if (and (string-match "^-?[0-9]*\\.?[0-9]*$" string)
            (= (length (substring string (match-beginning 0)
 				 (match-end 0)))

lisp/org-publish.el

@@ -717,7 +717,7 @@ If :auto-sitemap is set, publish the sitemap too.
 If :makeindex is set, also produce a file theindex.org."
   (mapc
    (lambda (project)
-     ;; Each project uses it's own cache file:
+     ;; Each project uses its own cache file:
      (org-publish-initialize-cache (car project))
      (let*
 	 ((project-plist (cdr project))