DOC: Fix typo. (#11813)

you could could be I/O bottlenecked.
TO:
you could be I/O bottlenecked.

tensorflow/docs_src/performance/performance_guide.md

@@ -104,7 +104,7 @@ with tf.device('/cpu:0'):
 Under some circumstances, both the CPU and GPU can be starved for data by the
 I/O system. If you are using many small files to form your input data set, you
 may be limited by the speed of your filesystem. If your training loop runs
-faster when using SSDs vs HDDs for storing your input data, you could could be
+faster when using SSDs vs HDDs for storing your input data, you could be
 I/O bottlenecked.
 
 If this is the case, you should pre-process your input data, creating a few