[range.elements.iterator] Add missing 'friend' for operator-. (#4158)

Author: jensmaurer

source/ranges.tex

@@ -6678,7 +6678,7 @@
 
 \indexlibrarymember{operator-}{elements_view::iterator}%
 \begin{itemdecl}
-constexpr @\exposid{iterator}@ operator-(const @\exposid{iterator}@& x, difference_type y)
+friend constexpr @\exposid{iterator}@ operator-(const @\exposid{iterator}@& x, difference_type y)
   requires @\libconcept{random_access_range}@<@\exposid{Base}@>;
 \end{itemdecl}
 
@@ -6690,7 +6690,7 @@
 
 \indexlibrarymember{operator-}{elements_view::iterator}%
 \begin{itemdecl}
-constexpr difference_type operator-(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
+friend constexpr difference_type operator-(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
   requires @\libconcept{random_access_range}@<@\exposid{Base}@>;
 \end{itemdecl}