install: help: remove duplicate info (#64)

Removes duplicated "(multple values allowed)" information from `poetry
help install`:

    % poetry install -h
    Usage:
    …
      -E, --extras=EXTRAS   Extra sets of dependencies to install
      (multiple values allowed). (multiple values allowed)

poetry/console/commands/install.py

@@ -9,8 +9,7 @@ class InstallCommand(VenvCommand):
         { --no-dev : Do not install dev dependencies. }
         { --no-dev : Do not install dev dependencies. }
         { --dry-run : Outputs the operations but will not execute anything
         { --dry-run : Outputs the operations but will not execute anything
                       (implicitly enables --verbose). }
                       (implicitly enables --verbose). }
-        { --E|extras=* : Extra sets of dependencies to install
-                         (multiple values allowed). }
+        { --E|extras=* : Extra sets of dependencies to install. }
     """
     """
 
 
     help = """The <info>install</info> command reads the <comment>pyproject.toml</> file from
     help = """The <info>install</info> command reads the <comment>pyproject.toml</> file from