Revert "change default sh code block shell to bash"

This reverts commit 1f498c93f297cf39f9afc609433eb764c1480f9e.

lisp/ob-sh.el

@@ -38,7 +38,7 @@
 
 (defvar org-babel-default-header-args:sh '())
 
-(defvar org-babel-sh-command "bash"
+(defvar org-babel-sh-command "sh"
   "Command used to invoke a shell.
 This will be passed to  `shell-command-on-region'")