typo: the're -> they're (#410)

concurrency.md

@@ -178,7 +178,7 @@ but without a name (unsurprisingly). You can see one above in the body of the
 
 Anonymous functions have a number of features which make them useful, two of
 which we're using above. Firstly, they can be executed at the same time that
-the're declared - this is what the `()` at the end of the anonymous function is
+they're declared - this is what the `()` at the end of the anonymous function is
 doing. Secondly they maintain access to the lexical scope they are defined in -
 all the variables that are available at the point when you declare the anonymous
 function are also available in the body of the function.