fix: documentation example to configure producer

[yordis]

fixes #296

[josevalim]

:green_heart: :blue_heart: :purple_heart: :yellow_heart: :heart:

[coveralls]

Pull Request Test Coverage Report for Build a370a4a63c8253d7eb8191a86ecd9fd6e5bd9521-PR-297

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 94.194%

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Files with Coverage Reduction	New Missed Lines	%
lib/broadway/topology/terminator.ex	1	92.86%
<!--	Total:	1		-->

Totals
Change from base Build 7581858b4b3b3df7694779a8b465200b6f4e01eb:	-0.2%
Covered Lines:	584
Relevant Lines:	620

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💛 - Coveralls