update process manager implementation examples

Author: bgrayburn

examples/hivemind/devenv.nix

@@ -1,7 +1,7 @@
 { pkgs, ... }:
 
 {
-  process.implementation = "hivemind";
+  process.manager.implementation = "hivemind";
   processes.foo.exec = "echo foo; sleep inf";
   processes.bar.exec = "echo bar; sleep inf";
 }

examples/overmind/devenv.nix

@@ -1,7 +1,7 @@
 { pkgs, ... }:
 
 {
-  process.implementation = "overmind";
+  process.manager.implementation = "overmind";
   processes.foo.exec = "echo foo; sleep inf";
   processes.bar.exec = "echo bar; sleep inf";
 }