Fix typo in concepts documentation
diff --git a/docs/concepts.md b/docs/concepts.md
index 74e1ca3..28a47b4 100644
--- a/docs/concepts.md
+++ b/docs/concepts.md
@@ -544,7 +544,7 @@
3. Perform any synchronization and error checking that may be needed
depending on application.
-For example UDP packets already fullfill all the requirements, and TCP
+For example UDP packets already fulfill all the requirements, and TCP
streams typically only need a way to identify the message length and
type. Lower level interfaces such as serial ports may need a more robust
frame format, such as HDLC (high-level data link control).
