Update ca2249.md and fix typo in the example code

[bforget163]

Update ca2249.md and fix typo in the example code

The code example is incorrect the variable name should be str instead of string.

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docs/fundamentals/code-analysis/quality-rules/ca2249.md	CA2249: Consider using String.Contains instead of String.IndexOf

[bforget163]

@dotnet-policy-service agree

[BillWagner]

Thanks for spotting this and fixing it @bforget163

I'll now. You should see the changes on the live site in the next day or two on our regular publishing cycle. Thanks again.