Fix MIX_INSTALL_DIR docs (#11338)

Fix accidentally removed part of the sentence.

Author: wojtekmach

lib/mix/lib/mix.ex

@@ -302,6 +302,7 @@ defmodule Mix do
     * `MIX_HOME` - path to Mix's home directory, stores configuration files and scripts used by Mix
       (default: `~/.mix`)
     * `MIX_INSTALL_DIR` - (since v1.12.0) specifies directory where `Mix.install/2` keeps
+       install cache
     * `MIX_INSTALL_FORCE` - (since v1.13.0) runs `Mix.install/2` with empty install cache
     * `MIX_PATH` - appends extra code paths
     * `MIX_QUIET` - does not print information messages to the terminal