Backport commit c68cce94c from Emacs

; Fix typo in error messages (Bug#26034)
c68cce94c46140f2ad1411550427d3cc2658ec02
Christian Garbs
Mon Apr 17 07:07:12 2017 +0300

TINYCHANGE

lisp/ob-C.el

@@ -278,12 +278,12 @@ its header arguments."
 (defun org-babel-prep-session:C (_session _params)
   "This function does nothing as C is a compiled language with no
 support for sessions"
-  (error "C is a compiled languages -- no support for sessions"))
+  (error "C is a compiled language -- no support for sessions"))
 
 (defun org-babel-load-session:C (_session _body _params)
   "This function does nothing as C is a compiled language with no
 support for sessions"
-  (error "C is a compiled languages -- no support for sessions"))
+  (error "C is a compiled language -- no support for sessions"))
 
 ;; helper functions