Standardize mix escript task's shortdoc (#4984)

Correct verb tense, and remove trailing period.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: eksperimental

lib/mix/lib/mix/tasks/escript.ex

@@ -1,7 +1,7 @@
 defmodule Mix.Tasks.Escript do
   use Mix.Task
 
-  @shortdoc "List installed escripts."
+  @shortdoc "Lists installed escripts"
 
   @moduledoc ~S"""
   Lists all installed escripts.