Fix typos and messages system tense

[benekex2]

Resolves #4129

I want to merge this change because...

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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. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #4168 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4168      +/-   ##
==========================================
+ Coverage    91.7%   91.72%   +0.01%     
==========================================
  Files         278      278              
  Lines       15485    15485              
  Branches     1506     1506              
==========================================
+ Hits        14201    14203       +2     
+ Misses        882      881       -1     
+ Partials      402      401       -1

Impacted Files	Coverage Δ
saleor/payment/utils.py	91.5% <0%> (+0.94%)	⬆️

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