Fix typo (tensorflow#1329)

Author: hsm207

tensor2tensor/layers/common_attention.py

@@ -423,7 +423,7 @@ def add_timing_signal_1d(x,
   memory inputs to attention.
 
   The use of relative position is possible because sin(x+y) and cos(x+y) can be
-  experessed in terms of y, sin(x) and cos(x).
+  expressed in terms of y, sin(x) and cos(x).
 
   In particular, we use a geometric sequence of timescales starting with
   min_timescale and ending with max_timescale.  The number of different