#252 Documentation error for associative tries

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Example 7.1.2 on page 338 of Manual 1 (XSB 4.0) contains multiple errors:

• Associative tries are created with type(pras), not type(assoc).
• Contrary to the example, trie_insert fails if the key already exists; the value is not updated. This behavior is correctly described in section 7.3.