README: fix usage instructions

The usage instructions suggest using ',,f' to create the table, however, doing this before the table has been created with `,,c` produces an error:

```
Error detected while processing function ReflowTable:                                                                                                                                                                                                                                                                          
line    7:                                                                                                                                                                                                                                                                                                                     
Traceback (most recent call last):                                                                                                                                                                                                                                                                                             
  File "<string>", line 3, in <module>                                                                                                                                                                                                                                                                                         
  File "<string>", line 289, in reflow_table                                                                                                                                                                                                                                                                                   
  File "<string>", line 204, in get_column_widths_from_border_spec                                                                                                                                                                                                                                                             
RuntimeError: Cannot reflow this table. Top table border not found.                                                                                                                                                                                                                                                            
Press ENTER or type command to continue    
```

Updated the usage instructions to create the table with `,,c` first and show an example of using `,,f` to reflow the table after it's been modified.