Update nn_pubsub.adoc (#1102)

Typo of initial
diff --git a/doc/nn_pubsub.adoc b/doc/nn_pubsub.adoc
index b932b1a..55ef462 100644
--- a/doc/nn_pubsub.adoc
+++ b/doc/nn_pubsub.adoc
@@ -28,7 +28,7 @@
 nn_setsockopt (s, NN_SUB, NN_SUB_SUBSCRIBE, "Hello", 5);
 ----
 
-Will match any message with intial 5 bytes being "Hello", for example,
+Will match any message with initial 5 bytes being "Hello", for example,
 message "Hello, World!" will match.
 
 Topic with zero length matches any message.