Fix path of dput.cf

Philippe SWARTVAGHERrequested to merge
phsw/debexpo:fix-dput-conf-path into live

Since dput 1.1.1, configuration file of dput has to be in $HOME/.config/dput/ (see https://salsa.debian.org/debian/dput/-/blob/main/debian/NEWS). This PR fixes the path in the instructions displayed on the page https://mentors.debian.net/intro-maintainers/.

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