Fix the job summary table example

This updates the example of adding a table to a job summary using
core.summary.addTable() to be correct. It also changes the data row to
use strings instead of objects to show both ways that cell data can be
defined. This mirrors the job summary announcement blog post.

packages/core/README.md

@@ -451,17 +451,17 @@ For example
 ```typescript
 
 const tableData = [
-  {data: 'Header1', header: true},
-  {data: 'Header2', header: true},
-  {data: 'Header3', header: true},
-  {data: 'MyData1'},
-  {data: 'MyData2'},
-  {data: 'MyData3'}
+  [
+    {data: 'Header1', header: true},
+    {data: 'Header2', header: true},
+    {data: 'Header3', header: true}
+  ],
+  ['MyData1', 'MyData2', 'MyData3']
 ]
 
 // Add an HTML table
-core.summary.addTable([tableData])
-// Output: <table><tr><th>Header1</th><th>Header2</th><th>Header3</th></tr><tr></tr><td>MyData1</td><td>MyData2</td><td>MyData3</td></tr></table>
+core.summary.addTable(tableData)
+// Output: <table><tr><th>Header1</th><th>Header2</th><th>Header3</th></tr><tr><td>MyData1</td><td>MyData2</td><td>MyData3</td></tr></table>
 
 ```