Update SafeAppProvider references in README.md

[fmrsabino]

@gnosis.pm/safe-apps-provider exports SafeAppProvider and not SafeAppsProvider so I updated the README to reflect that

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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