Correct comment in NativeSessionStorage regarding session.save_handler

It's save_handler, not save_handlers.
merged branch igorw/session-save-handler-comment (PR #6922)

This PR was merged into the 2.1 branch.

Commits
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d6c0455 Correct comment in NativeSessionStorage regarding session.save_handler

Discussion
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Correct comment in NativeSessionStorage regarding session.save_handler

It's save_handler, not save_handlers.