Fix typo

src/content/chapter5_advanced_features/lesson07_multi_target_externals/en.html

@@ -8,7 +8,7 @@
 </p>
 <p>
   You should try to implement functions for all targets, but this isn't always
-  possible due to incompatibilities in how IO and concurreny works in Erlang and
+  possible due to incompatibilities in how IO and concurrency works in Erlang and
   JavaScript. With Erlang concurrent IO is handled transparently by the runtime,
   while in JavaScript concurrent IO requires the use of promises or callbacks.
   If your code uses the Erlang style it is typically not possible to implement