fix(react): make instructions elements ids unique

[lex111]

What kind of change does this pull request introduce?

The instructions elements ids in each code block must be unique, otherwise you will get the following HTML errors if you use more than one code block on web page:

Duplicate ID “enter-instructions”.
The id attribute is used to identify a single element within a document, and is required to be unique. Check the document for repeated IDs.

What is the current behavior?

When I try to check the site with the W3C Validator, I get the "Duplicate ID" errors (see https://validator.w3.org/nu/?doc=https%3A%2F%2Fbeta.reactjs.org%2Flearn for example).

What is the new behavior?

No any "Duplicate ID" errors when validator checks web page that contains multiple code blocks.

What steps did you take to test this? This is required before we can merge, make sure to test the flow you've updated.

1. Include two code blocks on same web page
2. Try to validate final HTML markup on https://validator.w3.org/
3. Make sure that no "Duplicate ID" errors such have been detected

Checklist

• Documentation
• Ready to be merged
  

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[danilowoz]

ohh nice catch! It looks good to me