docs(config): fix default value for install_command

The `install_command` config is documented as having the default value:

  python -I -m pip install <opts> <packages>

The last two arguments are not substituable and are thus passed as is
(eg as packages to install) resulting in:

  py3-test: install_deps> python -I -m pip install -v '<opts>'
  '<packages>' '.[test]'
  ERROR: Invalid requirement: '<opts>'

Adjust the documentation to use the replaceable variables: `{opts}` and
`{packages}`.

docs/changelog/3126.docs.rst

@@ -0,0 +1 @@
+Fix default value for `install_command` - by :user:`hashar`.

docs/config.rst

@@ -795,7 +795,7 @@ Pip installer
 
 .. conf::
    :keys: install_command
-   :default: python -I -m pip install <opts> <packages>
+   :default: python -I -m pip install {opts} {packages}
    :version_added: 1.6
 
    Determines the command used for installing packages into the virtual environment; both the package under test and its