project-tree: Clarify "Reveal in Project" label

Merged Vanadiae requested to merge Vanadiae/gnome-builder:clarify-reveal-in-project-label into master

Currently it is unclear where this "Project" leaves in the UI, and hence what this menu item will do. In fact, this item selects the currently shown file in the project tree, located on the left sidebar.

So this commit clarifies this label text, by simply specifying it's the "Project Tree", which is clearer.

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