doc: Close code span correctly

The code span is closed with a straight quote instead of the correct
back tick being used.

PR-URL: https://github.com/iojs/io.js/pull/814
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>

doc/api/child_process.markdown

@@ -177,7 +177,7 @@ Example:
 
 ### child.connected
 
-* {Boolean} Set to false after `.disconnect' is called
+* {Boolean} Set to false after `.disconnect` is called
 
 If `.connected` is false, it is no longer possible to send messages.