Fix too long line in the description manual

Manual/Description/system.stex

@@ -1881,7 +1881,7 @@ the current theory.
 
 \noindent Evaluating
  \ml{new\_definition("$name$", \holquote{$c\ v_1\ \cdots\ v_n\ =\ t$})},
-declares the sequent
+declares the sequent\\
 \ml{(\lb\rb{},$c = \lambda v_1\ \cdots\ v_n.\  t$)} to be a constant definition
 \index{definitions, adding to HOL logic@definitions, adding to \HOL{} logic}
 of the current theory. The name associated with the definition in