Strange placeholder behaviour in input field

Replace `it’s` with `it is` because the apostrophe is rendered with a big space with the configured `AvenirNext-Regular` font-family. This is the only occurrence of `it’s` in the whole UI, everywhere else `it is` is used. Perhaps for certain reasons.

Signed-off-by: Volker Theile <vtheile@suse.com>

src/routes/backingImage/CreateBackingImage.js

@@ -209,7 +209,7 @@ const modal = ({
                 required: false,
               },
             ],
-          })(<Input placeholder="Ask Longhorn to validate the SHA512 checksum if it’s specified here." />)}
+          })(<Input placeholder="Ask Longhorn to validate the SHA512 checksum if it is specified here." />)}
         </FormItem>
       </Form>
     </ModalBlur>