Fix a minor typo in codegen.rst

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[] are either individually buildable or squashed [] have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate) [] have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places. [] add a testcase to the testsuite. [] if your MR affects library interfaces (e.g. changes base) please add the ~"user facing" label. [] updates the users guide if applicable [*] mentions new features in the release notes for the next release

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