Fix small typos in comments.
@@ -23,7 +23,7 @@ Python's expensive I/O path.
5) The data processing pipeline then waits to read sort options
from a second pipe.
-6) The Python control process reads the header line from the dirst pipe
+6) The Python control process reads the header line from the first pipe.
7) The Python control process feeds the header line to KgtkReader.
