chore(readme): update sponsor image sources

[AnaelBonnafous]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe: chore(readme)

What is the current behavior?

Sponsors images are not showing in readme files

Issue Number: N/A

What is the new behavior?

Sponsors images are now correctly linked to sources from nestjs.com website and are showing in readme files of nest reposity

Does this PR introduce a breaking change?

• Yes
• No

Other information

Linked to PR 98 in nestjs.com repository : nestjs/nestjs.com#98

[coveralls]

Pull Request Test Coverage Report for Build 0d408664-1ecc-4002-8deb-97fae2a273bc

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.124%

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Totals
Change from base Build e56f6d99-d9bc-426a-b9ef-ed11406d7e1c:	0.0%
Covered Lines:	6737
Relevant Lines:	7313

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💛 - Coveralls

[kamilmysliwiec]

lgtm