Update dbal.rst

The SchemaTool throws an error if mapping_types are configured under default.connection. If configured under dbal everything works fine.
minor #3944 Update dbal.rst (bpiepiora)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #3944).

Discussion
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Update dbal.rst

The SchemaTool throws an error if mapping_types are configured under default.connection. If configured under dbal everything works fine.

Commits
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bcfa370 Update dbal.rst