console: Fix typo in docs for `self add` (#7848)

src/poetry/console/commands/self/add.py

@@ -16,7 +16,7 @@ class SelfAddCommand(SelfCommand, AddCommand):
         if o.name in {"editable", "extras", "source", "dry-run", "allow-prereleases"}
     ]
     help = f"""\
-The <c1>self add</c1> command installs additional package's to Poetry's runtime \
+The <c1>self add</c1> command installs additional packages to Poetry's runtime \
 environment.
 
 This is managed in the <comment>{SelfCommand.get_default_system_pyproject_file()}</> \