get_maintainer.py: fix spelling mistake

OSError is spelled incorrectly, causing the following error:

 $ ./scripts/get_maintainer.py core/arch/arm/plat-imx/
 Traceback (most recent call last):
   File "./scripts/get_maintainer.py", line 102, in split_patchset
     f = open(patchset, "r")
 IsADirectoryError: [Errno 21] Is a directory: 'core/arch/arm/plat-imx/'

 During handling of the above exception, another exception occurred:

 Traceback (most recent call last):
   File "./scripts/get_maintainer.py", line 292, in <module>
     main()
   File "./scripts/get_maintainer.py", line 242, in main
     patches = split_patchset(arg)
   File "./scripts/get_maintainer.py", line 103, in split_patchset
     except OsError:
 NameError: name 'OsError' is not defined

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/scripts/get_maintainer.py b/scripts/get_maintainer.py
index 645db61..6600c44 100755
--- a/scripts/get_maintainer.py
+++ b/scripts/get_maintainer.py
@@ -100,7 +100,7 @@
     f = None
     try:
         f = open(patchset, "r")
-    except OsError:
+    except OSError:
         return []
     for line in f:
         match = re.search(PATCH_START, line)