map alternate code blocks shell names to sh-mode

* lisp/org-src.el (org-src-lang-modes): Map alternate shell names to
  sh-mode.

lisp/org-src.el

@@ -181,7 +181,7 @@ but which mess up the display of a snippet in Org exported files.")
   '(("ocaml" . tuareg) ("elisp" . emacs-lisp) ("ditaa" . artist)
     ("asymptote" . asy) ("dot" . fundamental) ("sqlite" . sql)
     ("calc" . fundamental) ("C" . c) ("cpp" . c++) ("C++" . c++)
-    ("screen" . shell-script))
+    ("screen" . shell-script) ("shell" . sh) ("bash" . sh))
   "Alist mapping languages to their major mode.
 The key is the language name, the value is the string that should
 be inserted as the name of the major mode.  For many languages this is