Update io_developers_guide.rst

fixed my own grammar mistake

Author: zm711

doc/source/io_developers_guide.rst

@@ -52,7 +52,7 @@ To use these you need to upload some sample data files at `gin-gnode`_. They wil
 These tests:
 
   * check for compliance with the schema: hierarchy, attribute types, ...
-  * For IO modules confirms they able to both write and read data; they compare a generated dataset with the same data after a write/read cycle.
+  * For IO modules confirm they able to both write and read data; they compare a generated dataset with the same data after a write/read cycle.
 
 The test scripts download all files from `gin-gnode`_ and store them locally in ``/tmp/files_for_tests/``.
 Subsequent test runs use the previously downloaded files, rather than trying to download them each time.