Update dynamic_form_modification.rst
No need to set "multiple" and "expanded" to false as the field's type is "entity".
Update dynamic_form_modification.rst
query_builder's callback must return a Doctrine\ORM\QueryBuilder

calling a repository in a "query_builder" callback would result in an exception saying:
Expected argument of type "Doctrine\ORM\QueryBuilder", "array" given
addde a line breaks after the first word that crossed the 72th character
[#2935] Enhancing note about using a repository to build a QB or to just call a function return the QB