Export: Use correct context for the `Posts` label on the Export screen.

The string refers to the post type name specifically, not to a generic term to identify a publication on WordPress.

Props audrasjb, jdy68.
Fixes #49306.
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git-svn-id: https://core.svn.wordpress.org/trunk@47072 1a063a9b-81f0-0310-95a4-ce76da25c4cd