Grammar correction in ReactDOMInput.js warning (#6657)

Changed "a uncontrolled input" to "an uncontrolled input".
(cherry picked from commit 393a179)

src/renderers/dom/client/wrappers/ReactDOMInput.js

@@ -169,7 +169,7 @@ var ReactDOMInput = {
       ) {
         warning(
           false,
-          '%s is changing a uncontrolled input of type %s to be controlled. ' +
+          '%s is changing an uncontrolled input of type %s to be controlled. ' +
           'Input elements should not switch from uncontrolled to controlled (or vice versa). ' +
           'Decide between using a controlled or uncontrolled input ' +
           'element for the lifetime of the component. More info: https://fb.me/react-controlled-components',

src/renderers/dom/client/wrappers/__tests__/ReactDOMInput-test.js

@@ -488,7 +488,7 @@ describe('ReactDOMInput', function() {
     ReactDOM.render(<input type="text" value="controlled" />, container);
     expect(console.error.argsForCall.length).toBe(1);
     expect(console.error.argsForCall[0][0]).toContain(
-      'A component is changing a uncontrolled input of type text to be controlled. ' +
+      'A component is changing an uncontrolled input of type text to be controlled. ' +
       'Input elements should not switch from uncontrolled to controlled (or vice versa). ' +
       'Decide between using a controlled or uncontrolled input ' +
       'element for the lifetime of the component. More info: https://fb.me/react-controlled-components'
@@ -530,7 +530,7 @@ describe('ReactDOMInput', function() {
     ReactDOM.render(<input type="checkbox" checked={true} />, container);
     expect(console.error.argsForCall.length).toBe(1);
     expect(console.error.argsForCall[0][0]).toContain(
-      'A component is changing a uncontrolled input of type checkbox to be controlled. ' +
+      'A component is changing an uncontrolled input of type checkbox to be controlled. ' +
       'Input elements should not switch from uncontrolled to controlled (or vice versa). ' +
       'Decide between using a controlled or uncontrolled input ' +
       'element for the lifetime of the component. More info: https://fb.me/react-controlled-components'
@@ -572,7 +572,7 @@ describe('ReactDOMInput', function() {
     ReactDOM.render(<input type="radio" checked={true} />, container);
     expect(console.error.argsForCall.length).toBe(1);
     expect(console.error.argsForCall[0][0]).toContain(
-      'A component is changing a uncontrolled input of type radio to be controlled. ' +
+      'A component is changing an uncontrolled input of type radio to be controlled. ' +
       'Input elements should not switch from uncontrolled to controlled (or vice versa). ' +
       'Decide between using a controlled or uncontrolled input ' +
       'element for the lifetime of the component. More info: https://fb.me/react-controlled-components'