package/exim: fix typo in comment

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 55ef42e090b39fdf129b7e5e218019e0156fe166)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/exim/exim.mk

@@ -20,7 +20,7 @@ define exim-config-change # variable-name, variable-value
 		$(@D)/Local/Makefile
 endef
 
-# Comment-out a variable. Has no effect if it does not exits.
+# Comment-out a variable. Has no effect if it does not exist.
 define exim-config-unset # variable-name
 	$(SED) 's,^\([[:space:]]*$1[[:space:]]*=.*$$\),# \1,' \
 		$(@D)/Local/Makefile