typo fixing (#1368)

Summary:
Pull Request resolved: #1368

typo fixing

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: esantorella

Differential Revision: D38988092

fbshipit-source-id: 97e0d6e271df63b67d8e6fff1a69f7b2c0326ea9

Author: ItsMrLin

botorch/models/pairwise_gp.py

@@ -71,7 +71,7 @@ class PairwiseGP(Model, GP):
     (implicitly setting it to 1) and use ScaleKernel to scale the function.
 
     In the example below, the user/decision maker has stated that they prefer
-    the first item over the second item and the third item over the first item,
+    the first item over the second item and the third item over the second item,
     generating comparisons [0, 1] and [2, 1].
 
     Example: