change ~coq-mathcomp-~ to ~rocq-mathcomp-~

[affeldt-aist]

math-comp.github.io/installation.org

Line 36 in 0c58891

opam install coq-mathcomp-ssreflect

"At some point, we should recommend rocq-mathcomp, since coq-mathcomp is just a compat metapackage." @proux01