docs(bylabeltext): Fix capitalization (#865)

* Update bylabeltext.mdx

Updated the example code for React and Cypress so it works properly.

* Update bylabeltext.mdx

Updated the examples so they are all capitalized the same way.

Author: ruhollahh

docs/queries/bylabeltext.mdx

@@ -50,7 +50,7 @@ The example below will find the input node for the following DOM structures:
 // aria-label attributes
 // Take care because this is not a label that users can see on the page,
 // so the purpose of your input must be obvious to visual users.
-<input aria-label="username" />
+<input aria-label="Username" />
 ```
 
 <Tabs defaultValue="native" values={[ { label: 'Native', value: 'native', }, {
@@ -71,14 +71,14 @@ import { render, screen } from '@testing-library/react'
 
 render(<Login />)
 
-const inputNode = screen.getByLabelText('username')
+const inputNode = screen.getByLabelText('Username')
 ```
 
   </TabItem>
   <TabItem value="cypress">
 
 ```js
-cy.findByLabelText('username').should('exist')
+cy.findByLabelText('Username').should('exist')
 ```
 
   </TabItem>