ensure instructions also work with opam >= 2.1

Thanks to Karoliine Holter for the suggestion.

html/get-frama-c.html

@@ -66,7 +66,7 @@ sudo apt install make m4 gcc opam
 # 2. opam setup
 opam init --disable-sandboxing --shell-setup
 eval $(opam env)
-opam install -y depext
+opam install -y opam-depext
 
 # 3. Install graphical dependencies
 opam depext --install -y lablgtk3 lablgtk3-sourceview3