chore(web.tex): also change to amsart (#485)

for consistency with the printed version

Author: grunweg

blueprint/src/web.tex

@@ -1,7 +1,7 @@
 % This file makes the web version of the blueprint.
 % This file can be built locally.
 
-\documentclass{amsbook}
+\documentclass{amsart}
 
 \usepackage{amsmath,amsfonts,amsthm,amssymb}
 \usepackage{graphicx}