To get rid of the square corner on the left size of the white text box above add this rounded shape on the left.

This is a text box with the background fill set to white.  The Inset Margin is set to 10 so as not to interfere with the rounded shape added to the left to complete the rounded corner.

This text remains as text in the published site.

Text in box

The text can be edited at this stage in iWeb. The overall combination is converted  to an image upon publication.

The text cannot be edited in iWeb once it’s been added into the 2nd  shape.

Text Within Shapes Within Other Shapes

If the text was added to the 1st shape instead of a text box in the first shape it would be editable as shown below

Using Multiple Shapes & Text Boxes