api: fix typo in README

Fixes #13690.

Change-Id: I3b9b993a2e7ecf07bab7d1935d4c83a86bc6ba3a
Reviewed-on: https://go-review.googlesource.com/18054Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

api/README

 Files in this directory are data for Go's API checker ("go tool api", in src/cmd/api).
 
-Each file is a list of of API features, one per line.
+Each file is a list of API features, one per line.
 
 go1.txt (and similarly named files) are frozen once a version has been
 shipped. Each file adds new lines but does not remove any.