fix typo

R=gri, iant
CC=golang-dev
https://golang.org/cl/183160

doc/go_spec.html

@@ -696,7 +696,7 @@ underlying array.
 </p>
 <p>
 A slice, once initialized, is always associated with an underlying
-array that holds its elements.  A slice therfore shares storage
+array that holds its elements.  A slice therefore shares storage
 with its array and with other slices of the same array; by contrast,
 distinct arrays always represent distinct storage.
 </p>