Use consistent indicative grammar mood in command-line help (#1369)

* Update about.py

* Update add.py

* Update update.py

poetry/console/commands/about.py

@@ -3,7 +3,7 @@ from .command import Command
 
 
 class AboutCommand(Command):
 class AboutCommand(Command):
     """
     """
-    Short information about Poetry.
+    Shows information about Poetry.
 
 
     about
     about
     """
     """

poetry/console/commands/add.py

@@ -4,7 +4,7 @@ from .env_command import EnvCommand
 
 
 class AddCommand(EnvCommand, InitCommand):
 class AddCommand(EnvCommand, InitCommand):
     """
     """
-    Add a new dependency to <comment>pyproject.toml</>.
+    Adds a new dependency to <comment>pyproject.toml</>.
 
 
     add
     add
         { name* : Packages to add. }
         { name* : Packages to add. }

poetry/console/commands/update.py

@@ -3,7 +3,7 @@ from .env_command import EnvCommand
 
 
 class UpdateCommand(EnvCommand):
 class UpdateCommand(EnvCommand):
     """
     """
-    Update dependencies as according to the <comment>pyproject.toml</> file.
+    Updates dependencies as according to the <comment>pyproject.toml</> file.
 
 
     update
     update
         { packages?* : The packages to update. }
         { packages?* : The packages to update. }