fix: Use the correct default minimium password length mentioned in in…

…stall operation document

The default minimum password length  mentioned in edx-platform/lms/envs/common.py is 8 characters but at install operation document you see 2.
closes #2119

en_us/install_operations/source/configuration/password.rst

@@ -35,7 +35,7 @@ under the ``AUTH_PASSWORD_VALIDATORS`` setting::
   -   NAME: django.contrib.auth.password_validation.UserAttributeSimilarityValidator
   -   NAME: common.djangoapps.util.password_policy_validators.MinimumLengthValidator
         OPTIONS:
-          min_length: 2
+          min_length: 8
   -   NAME: common.djangoapps.util.password_policy_validators.MaximumLengthValidator
         OPTIONS:
           max_length: 75