Fix copy-paste error

[apapillon]

Just a little copy/paste error in inputGroup example

<InputGroup size="lg">
        <InputGroup.Text id="inputGroup-sizing-lg">Large</InputGroup.Text>
        <Form.Control
          aria-label="Large"
          aria-describedby="inputGroup-sizing-sm"    >    aria-describedby="inputGroup-sizing-lg"
        />
</InputGroup>

In area-describedby, change sm to lg