TextBook: Center image captions

textbook/assets/css/blocks.css

@@ -25,6 +25,7 @@ Description: Used to style Gutenberg Blocks.
 	font-size: 0.875rem;
 	font-weight: 400;
 	line-height: 1.65em;
+	text-align: center;
 }
 
 /* Video */