fix typo in figure

src/10_xray.md

@@ -217,8 +217,8 @@ def text_dict(text,pos,ref=('x1','y1')):
 annotations = [a.to_plotly_json() for a in fig["layout"]["annotations"]]
 annotations.append(text_dict('$\mathbf{a_1}$',(1.5,-0.5)))
 annotations.append(text_dict('$\mathbf{a_2}$',(0,1.3)))
-annotations.append(text_dict('$\mathbf{b_2}$',(0.8,-1),('x2','y2')))
-annotations.append(text_dict('$\mathbf{b_1}$',(-0.6,1.2),('x2','y2')))
+annotations.append(text_dict('$\mathbf{b_1}$',(0.8,-1),('x2','y2')))
+annotations.append(text_dict('$\mathbf{b_2}$',(-0.6,1.2),('x2','y2')))
 fig["layout"]["annotations"] = annotations