docs: fix translations update instructions

[igobranco]

closes #354

Removes from the README.md the incorrect instructions to update only some languages instead of all.

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[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f9dac31) 97.68% compared to head (e56b799) 97.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #355 +/- ## ======================================= Coverage 97.68% 97.68% ======================================= Files 77 77 Lines 6187 6187 ======================================= Hits 6044 6044 Misses 143 143 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `97.68% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#carryforward-flags-in-the-pull-request-comment) to find out more. Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx)

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[mphilbrick211]

@chennighan2u - another one for your review/merge!

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