Fix typo in clean_input method

[patrick-emmanuel]

I want to merge this change because...
Fixes typo in graphql/core/mutations

Screenshots

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. The changes are tested.
6. The code is documented (docstrings, project documentation).
7. GraphQL schema and type definitions are up to date.

[codecov]

Codecov Report

Merging #3100 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3100      +/-   ##
==========================================
+ Coverage   89.83%   89.86%   +0.03%     
==========================================
  Files         208      208              
  Lines       11178    11214      +36     
  Branches     1105     1108       +3     
==========================================
+ Hits        10042    10078      +36     
  Misses        802      802              
  Partials      334      334

Impacted Files	Coverage Δ
saleor/graphql/core/mutations.py	92.74% <ø> (ø)	⬆️
saleor/core/permissions.py	100% <0%> (ø)	⬆️
saleor/graphql/utils.py	94.64% <0%> (+0.41%)	⬆️
saleor/graphql/core/types/common.py	97.18% <0%> (+1.83%)	⬆️

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