fix: Update default Modal heading element to h1 (#3048)

* fix: Update default Modal heading element to h1

* add section check

Author: bxiao

components/modal/__tests__/modal.browser-test.jsx

@@ -234,6 +234,11 @@ describe('SLDSModal: ', function () {
 			modal = getModalNode(document.body);
 		});
 
+		it('adds the default h1 heading element', () => {
+			const header = modal.querySelector('section .slds-modal__header h1');
+			expect(header).to.not.be.null;
+		});
+
 		it('adds the footer', () => {
 			const footer = modal.querySelector('.slds-modal__footer');
 			expect(footer.className).to.include('slds-theme_default');

components/modal/index.jsx

@@ -449,15 +449,15 @@ class Modal extends React.Component {
 			headerContent = (
 				<div>
 					{this.props.toast}
-					<h2
+					<h1
 						className={classNames({
 							'slds-text-heading_small': this.isPrompt(),
 							'slds-text-heading_medium': !this.isPrompt(),
 						})}
 						id={`${this.getId()}-heading`}
 					>
 						{this.props.heading ? this.props.heading : this.props.title}
-					</h2>
+					</h1>
 					{this.props.tagline ? (
 						<p className="slds-m-top_x-small">{this.props.tagline}</p>
 					) : null}