[valarray.access] Remove superfluous whitespace (#7567)

Author: Andreas-Krug

source/numerics.tex

@@ -7499,7 +7499,7 @@
 
 \indexlibrarymember{operator[]}{valarray}%
 \begin{itemdecl}
-const T&  operator[](size_t n) const;
+const T& operator[](size_t n) const;
 T& operator[](size_t n);
 \end{itemdecl}