docs: fix a quantifiers

docs/derived.tex

@@ -16,7 +16,7 @@ We collect here some important and frequently used derived proof rules.
   {\prop * \Exists\var.\propB \provesIff \Exists\var. \prop * \propB}
 
   \infer{}
-  {\prop * \Exists\var.\propB \proves \Exists\var. \prop * \propB}
+  {\prop * \All\var.\propB \proves \All\var. \prop * \propB}
 
   \infer{}
   {\always(\prop*\propB) \provesIff \always\prop * \always\propB}