[iterator.requirements.general] Fix indexing around 'valid range'.

Author: jensmaurer

source/iterators.tex

@@ -609,7 +609,7 @@
 A sentinel \tcode{s} is called \defn{reachable from} an iterator \tcode{i} if
 and only if there is a finite sequence of applications of the expression
 \tcode{++i} that makes \tcode{i == s}. If \tcode{s} is reachable from \tcode{i},
-\range{i}{s} denotes a valid range.
+\range{i}{s} denotes a \defnadj{valid}{range}.
 
 \pnum
 A \defnadj{counted}{range} \countedrange{i}{n} is empty if \tcode{n == 0};
@@ -618,7 +618,7 @@
 starting with the element pointed to by \tcode{i} and up to but not including
 the element, if any, pointed to by
 the result of \tcode{n} applications of \tcode{++i}.
-A counted range \countedrange{i}{n} is valid if and only if \tcode{n == 0};
+A counted range \countedrange{i}{n} is \defnx{valid}{range!counted!valid} if and only if \tcode{n == 0};
 or \tcode{n} is positive, \tcode{i} is dereferenceable,
 and \countedrange{++i}{--n} is valid.