Replace unguaranteed file closing with 'with' statement in release_noter

As this will automatically close the open file no matter what; cf. [1].

[1] https://www.python.org/dev/peps/pep-0343

Change-Id: I1f8ce004763917014f23bb168fa63d954df56c13
diff --git a/tools/release_noter/release_noter.py b/tools/release_noter/release_noter.py
index eb00d5d..2702764 100644
--- a/tools/release_noter/release_noter.py
+++ b/tools/release_noter/release_noter.py
@@ -268,11 +268,10 @@
 
 
 def print_notes(commits, submodules):
-    md = open("release_noter.md", "w")
-    md.write("# Release Notes\n")
-    print_submodules(submodules, md)
-    print_commits(commits, md)
-    md.close()
+    with open("release_noter.md", "w") as md:
+        md.write("# Release Notes\n")
+        print_submodules(submodules, md)
+        print_commits(commits, md)
 
 
 if __name__ == "__main__":