Fixing a typo in the NumberPicker documentation.

/**

 * A widget that enables the user to select a number form a predefined range.

 * The widget presents an input filed and up and down buttons for selecting the

 * The widget presents an input field and up and down buttons for selecting the

 * current value. Pressing/long pressing the up and down buttons increments and

 * decrements the current value respectively. Touching the input filed shows a

 * decrements the current value respectively. Touching the input field shows a

 * scroll wheel, tapping on which while shown and not moving allows direct edit

 * of the current value. Sliding motions up or down hide the buttons and the

 * input filed, show the scroll wheel, and rotate the latter. Flinging is

 * input field, show the scroll wheel, and rotate the latter. Flinging is

 * also supported. The widget enables mapping from positions to strings such

 * that instead the position index the corresponding string is displayed.

 * <p>