doc: fix go_spec spelling error

R=rsc, gri
CC=golang-dev
https://golang.org/cl/2985041

doc/go_spec.html

@@ -4396,7 +4396,7 @@ BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
 <p>
 For a channel <code>c</code>, the built-in function <code>close(c)</code>
 marks the channel as unable to accept more values through a send operation;
-values sent to a closed channed are ignored.
+values sent to a closed channel are ignored.
 After calling <code>close</code>, and after any previously
 sent values have been received, receive operations will return
 the zero value for the channel's type without blocking.