Fix typo

`tpc` should be `tcp` here.
diff --git a/doc/nanocat.adoc b/doc/nanocat.adoc
index fd7b7df..267b1dc 100644
--- a/doc/nanocat.adoc
+++ b/doc/nanocat.adoc
@@ -162,7 +162,7 @@
 
 Send heartbeats to imaginary monitoring service:
 
-    nanocat --pub --connect tpc://monitoring.example.org -D"I am alive!" --interval 10
+    nanocat --pub --connect tcp://monitoring.example.org -D"I am alive!" --interval 10
 
 
 SEE ALSO