ox-publish: Correct docstring typo

* lisp/ox-publish.el (org-publish-project-alist): Correct typo in
  docstring.

lisp/ox-publish.el

@@ -110,7 +110,7 @@ Most properties are optional, but some should always be set:
     Directory (possibly remote) where output files will be
     published.
 
-If `:recursive' is non-nil files in sub-directors of
+If `:recursive' is non-nil files in sub-directories of
 `:base-directory' are considered.
 
 The `:exclude' property may be used to prevent certain files from