dvc: rename plot to plots (#3802)

* dvc: rename plot to plots

Follows `dvc params/metrics` convention and is needed in preparation for new
commands and some refactoring.

Part of #3409

* completion: rename plot -> plots

dvc/cli.py

@@ -27,7 +27,7 @@ from .command import (
     move,
     params,
     pipeline,
-    plot,
+    plots,
     remote,
     remove,
     repro,
@@ -74,7 +74,7 @@ COMMANDS = [
     version,
     update,
     git_hook,
-    plot,
+    plots,
 ]