Small readme example update (#1412)

in commit ea79cd8388 the`getRowProps` member of the headerGroup object changed into `getHeaderGroupProps`

README.md

@@ -481,7 +481,7 @@ function MyTable({ columns, data }) {
     <table {...getTableProps()}>
       <thead>
         {headerGroups.map(headerGroup => (
-          <tr {...headerGroup.getRowProps()}>
+          <tr {...headerGroup.getHeaderGroupProps()}>
             {headerGroup.headers.map(column => (
               <th {...column.getHeaderProps()}>{column.render('Header')}</th>
             ))}