Changed PHPDoc of Tax's getProductTaxRate to more accurate.

[stijnstroeve]

Questions	Answers
Branch?	develop
Description?	In the current version of the Tax class the method getProductTaxRate returns a Tax object according to the PHPDoc. I have changed this PHPDoc to return the type float.
Type?	bug fix
Category?	CO
BC breaks?	no
Deprecations?	no
How to test?	It does not really need testing because this only affects a PHPDoc.

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This change is 

[prestonBot]

Hello @stijnstroeve!

This is your first pull request on the PrestaShop project. Thank you, and welcome to this Open Source community!

[prestonBot]

Hi, thanks for this contribution!

I found some issues with the Pull Request description:

• Your pull request does not seem to fix any issue, consider creating one (see note below) and linking it by writing Fixes #1234.

Would you mind having a look at it? This will help us understand how interesting your contribution is, thank you very much!

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[Progi1984]

No need QA : it's PHPDoc.

Thanks @stijnstroeve