Fix type for helm-projectile-sources-list variable

[rrudakov]

According to the https://www.gnu.org/software/emacs/manual/html_node/elisp/Composite-Types.html list is a composite type and should be used when the number of elements and its types are known. For helm-projectile-sources-list the better option would be (repeat symbol).

emacs-29 introduces setopt macro to set customizable variables, it verifies variable type according to :type annotation and throw an error if type mismatches.

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