scripts/oe-selftest: fix typo

Change 'agains' to 'against'.

(From OE-Core rev: b72dba9cadb1ae0ee3feb184af1c5bb4a99f70e3)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-selftest

@@ -87,7 +87,7 @@ def logger_create():
 log = logger_create()
 
 def get_args_parser():
-    description = "Script that runs unit tests agains bitbake and other Yocto related tools. The goal is to validate tools functionality and metadata integrity. Refer to https://wiki.yoctoproject.org/wiki/Oe-selftest for more information."
+    description = "Script that runs unit tests against bitbake and other Yocto related tools. The goal is to validate tools functionality and metadata integrity. Refer to https://wiki.yoctoproject.org/wiki/Oe-selftest for more information."
     parser = argparse_oe.ArgumentParser(description=description)
     group = parser.add_mutually_exclusive_group(required=True)
     group.add_argument('-r', '--run-tests', required=False, action='store', nargs='*', dest="run_tests", default=None, help='Select what tests to run (modules, classes or test methods). Format should be: <module>.<class>.<test_method>')