[dune] [configure] Fix typo in default prefix setting.
[dune] [configure] Fix typo in default prefix setting.

Fix silly typo that created havoc in developer out-of-the-box setups.

maximedenes/coq,ppedrot/coq,ejgallego/coq,gares/coq,gares/coq,ejgallego/coq,Matafou/coq,coq/coq,ejgallego/coq,ppedrot/coq,SkySkimmer/coq,maximedenes/coq,Matafou/coq,JasonGross/coq,Zimmi48/coq,ppedrot/coq,coq/coq,JasonGross/coq,gares/coq,ppedrot/coq,coq/coq,SkySkimmer/coq,maximedenes/coq,coq/coq,ppedrot/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,maximedenes/coq,silene/coq,silene/coq,maximedenes/coq,JasonGross/coq,Zimmi48/coq,JasonGross/coq,Zimmi48/coq,Matafou/coq,gares/coq,Zimmi48/coq,silene/coq,gares/coq