Another error condition in parenthesis instead of at start of sentence

Addressing review comment by Hans.

chapters/functions.tex

@@ -452,8 +452,7 @@ \subsection{Positional or Named Input Arguments of Functions}\doublelabel{positi
 those other inputs will then be substituted into the default values
 (this process may be repeated if the default value for that input depend
 on another input). The default values for inputs may not depend on
-non-input variables in the function. Existence of any remaining unfilled slots is an error and the list of
-filled slots is used as the argument list for the call.
+non-input variables in the function.  The list of filled slots is used as the argument list for the call (it is an error if any unfilled slots still remain).
 
 Special purpose operators with function syntax defined in the
 specification may not be called with named arguments, unless otherwise