Add space between "p." and page number

This matches how page numbers are more typically written in e.g. citations.

src/sidebar/components/Annotation/AnnotationHeader.tsx

@@ -173,7 +173,7 @@ function AnnotationHeader({
               )}
               {pageNumber && (
                 <span className="text-grey-6" data-testid="page-number">
-                  {showDocumentInfo && ', '}p.{pageNumber}
+                  {showDocumentInfo && ', '}p. {pageNumber}
                 </span>
               )}
             </span>

src/sidebar/components/Annotation/test/AnnotationHeader-test.js

@@ -478,7 +478,7 @@ describe('AnnotationHeader', () => {
       const wrapper = createAnnotationHeader({ annotation });
       const pageNumber = wrapper.find('[data-testid="page-number"]');
       assert.isTrue(pageNumber.exists());
-      assert.equal(pageNumber.text(), 'p.11');
+      assert.equal(pageNumber.text(), 'p. 11');
     });
 
     it('should hide group name in sidebar', () => {