docs: replace getElementByText with getByText (#150)

Related to testing-library/dom-testing-library#82.

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**: Use getByText instead of getElementByText in a code example, which seems like it was a typo.

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**Why**: getElementByText is not exported by dom-testing-library as far as I'm aware, which could be confusing.

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* [x] Documentation
* [ ] Tests: N/A
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Author: nickserv

README.md

@@ -506,12 +506,12 @@ for a full list as well as default `eventProperties`.
 ```javascript
 import {render, fireEvent} from 'react-testing-library'
 
-const {getElementByText} = render(<Form />)
+const {getByText} = render(<Form />)
 
 // similar to the above example
 // click will bubble for React to see it
 const rightClick = {button: 2}
-fireEvent.click(getElementByText('Submit'), rightClick)
+fireEvent.click(getByText('Submit'), rightClick)
 // default `button` property for click events is set to `0` which is a left click.
 ```