Documentation: DocBook/Makefile comment typo

Fix a se for so typo.
Signed-off-by: NJim Davis <jim.epost@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -272,6 +272,6 @@ cleandocs:
 	$(Q)rm -rf $(call objectify, $(clean-dirs))
 
 # Declare the contents of the .PHONY variable as phony.  We keep that
-# information in a variable se we can use it in if_changed and friends.
+# information in a variable so we can use it in if_changed and friends.
 
 .PHONY: $(PHONY)