[book][translation] Fixing typo per @stof

Author: weaverryan

book/translation.rst

@@ -746,7 +746,7 @@ texts* and complex expressions:
  <h3>foo</h3>
  {% endtrans %}
 
- {% message = '<h3>foo</h3>' %}
+ {% set message = '<h3>foo</h3>' %}
 
  {# a variable translated via a filter is escaped by default #}
  {{ message | trans | raw }}