Need a space between function name and paren

Add a space between the function name and the opening parenthesis for
the parameters.  This is needed for `ext_diff` and `lie_diff`.  This
is required AFAICT by texinfo.

Adding the space fixes the issue where we had a warning that we had
HTML entries for "lie_diff(vector1,[vector2,form])" and
"ext_diff(form)" that were not in text entries.  However, there are
still two HTML entries not in text entries.  We no longer have a
warning that there are 2 text entries not in the the HTML entries.

Author: rtoy

doc/info/cartan.texi

@@ -87,12 +87,12 @@ vector on a form. The vector should be given on the left.
 @end deffn
 
 @anchor{ext_diff}
-@deffn {Function} ext_diff(@var{form})
+@deffn {Function} ext_diff (@var{form})
 The exterior derivative operator. It takes one argument, which should be a differential form.
 @end deffn
 
 @anchor{lie_diff}
-@deffn {Function} lie_diff(@var{vector1},[@var{vector2},@var{form}])
+@deffn {Function} lie_diff (@var{vector1},[@var{vector2},@var{form}])
 The Lie derivative operator. The first argument is a vector field. The
 second argument may be either a vector field or a differential form.
 @end deffn