[temp.param] Remove mistaken "opt".

This fixes an oversight in the paper.

Author: tkoeppe

source/templates.tex

@@ -266,7 +266,7 @@
 \begin{bnf}
 \nontermdef{type-tt-parameter}\br
   template-head type-parameter-key \opt{\terminal{...}} \opt{identifier}\br
-  template-head type-parameter-key \opt{identifier} \opt{type-tt-parameter-default}
+  template-head type-parameter-key \opt{identifier} type-tt-parameter-default
 \end{bnf}
 
 \begin{bnf}