[nix] Update ref for ocamlformat 0.15
[nix] Update ref for ocamlformat 0.15

Co-authored-by: Théo Zimmermann <e1d8818cf57db40c64760cc1689d30471aa2e406@gmail.com>

silene/coq,Zimmi48/coq,gares/coq,coq/coq,JasonGross/coq,ejgallego/coq,gares/coq,coq/coq,gares/coq,gares/coq,ejgallego/coq,gares/coq,JasonGross/coq,Zimmi48/coq,JasonGross/coq,ejgallego/coq,ejgallego/coq,silene/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq,SkySkimmer/coq,coq/coq,JasonGross/coq,SkySkimmer/coq,SkySkimmer/coq,coq/coq,silene/coq