Update leak-canary-plugin.md (#519)
Update leak-canary-plugin.md (#519)

Summary:
Unexpected semicolon in refWatcher sample code

copy pasting the sample code for refWatcher giving below error due to a semicolon in refWatcher code
![image](https://user-images.githubusercontent.com/10504808/63110022-25236b80-bfa8-11e9-8756-c95afc7d68a9.png)

## Changelog

remove the semicolon from refWatcher sample code
Pull Request resolved: https://github.com/facebook/flipper/pull/519

Reviewed By: danielbuechele

Differential Revision: D17527312

Pulled By: passy

fbshipit-source-id: c573736dec63de2e93113b56c24a7bab72bb0067

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