add session to the arguments of switch_page fn

[russHyde]

The text describes how the 'switch_page' function can be brought outside of the 'server' function if a 'session' argument is added to it.
But, the definition of 'switch_page' in the subsequent code block did not have a 'session' argument.
Hence, 'session' arg was added in (and passed to updateTabsetPanel).

Hope this makes sense, and I haven't misunderstood the original code.

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Also wondering whether updateTabsetPanel should be passed an explicit 'inputId = ...' rather than implicit 'input = ...'

[hadley]

Gah, this whole section needs to be re-written because updateTabsetPanel no longer needs the session argument.