fix(schema): use right type check for __modify_schema__

[PrettyWood]

Change Summary

As @therefromhere explains in #1552, __modify_schema__ was called for the schema field but also for the list schema because field.type_ would be the same in both cases.
Changed it to field.outer_type_

Related issue number

fix #1552

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1562 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1562   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3793      3793           
  Branches       752       752           
=========================================
  Hits          3793      3793           

Impacted Files	Coverage Δ
pydantic/schema.py	100.00% <100.00%> (ø)

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[therefromhere]

Just to confirm, this fixes the issue I was seeing, thanks! 👍

[samuelcolvin]

great, thanks.