Fix a typo in README file

[leandroh]

Fixes a typo in README.md Example

[sourcelevel-bot[bot]]

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[coveralls]

Pull Request Test Coverage Report for Build 315

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 85.924%

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Files with Coverage Reduction	New Missed Lines	%
lib/mongo_ecto.ex	5	84.68%
<!--	Total:	5		-->

Totals
Change from base Build 314:	0.0%
Covered Lines:	293
Relevant Lines:	341

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 315

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 85.924%

---

Files with Coverage Reduction	New Missed Lines	%
lib/mongo_ecto.ex	5	84.68%
<!--	Total:	5		-->

Totals
Change from base Build 314:	0.0%
Covered Lines:	293
Relevant Lines:	341

---

💛 - Coveralls

[ankhers]

Thank you!