Fix typo in comment in serializers.py

[Pairman]

Fix typo in StringSerializer's comment header, in serializers.py. "symple" is changed to "simple".

What do these changes do?

In aiocache/serializers/serializers.py, there's a typo in the comment header of the StringSerializer class. The typo "symple" is changed to "simple" in this commit.

Are there changes in behavior for the user?

No other changes than the comment header of StringSerializer visually shown in IDEs.

Related issue number

No related issues.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.76%. Comparing base (fe28c47) to head (bf71afb).
Report is 72 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #859   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files          36       36           
  Lines        3794     3797    +3     
=======================================
+ Hits         3785     3788    +3     
  Misses          9        9           

Flag	Coverage Δ
unit	99.76% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
aiocache/serializers/serializers.py	95.45% <ø> (ø)

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