Fix typo

spec/rule_list.tex

@@ -21,7 +21,7 @@ $\grT{(assume }\dots\grT{)}$ command.
 \proofsep& i.& \varphi &(\currule\; p_1, \dots, p_n)\; [a_1, \dots, a_n]\\
 \end{plList}
 \end{plContainer}
-where $\varphi$ is any well formula.
+where $\varphi$ is any well-formed formula.
 
 This rule can be used to express holes in the proof.  It can be used by
 solvers as a placeholder for proof steps that are not yet expressed