doc: fix name_id_format vs name_id_policy_format ambiguity

[bajnokk]

After 0c1873da1 in pysaml2, the ambiguity between the format in the of the and the in the metadata has been resolved. This change provides a followup in the SATOSA documentation and example.

Fixes: #415

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[c00kiemon5ter]

Thank you Kristof!