Update README.md

README.md

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 * Polypropylene
 * Steel
 * Stainless
-* Aluminum: Must be a formable series, like 3003.  <a href='https://www.onlinemetals.com/merchant.cfm?id=753&step=2&top_cat=60'>Online Metals</a>
+* Aluminum: Must be a formable series, like 1100 or 3003.  <a href='https://www.onlinemetals.com/merchant.cfm?id=753&step=2&top_cat=60'>Online Metals</a>
 
 
 ### Design tools