[over.sub] Fix typo: change oeprator to operator (#5140)

Author: nsajko

source/overloading.tex

@@ -3315,7 +3315,7 @@
 \begin{codeblock}
 struct X {
   Z operator[](std::initializer_list<int>);
-  Z oeprator[](auto...);
+  Z operator[](auto...);
 };
 X x;
 x[{1,2,3}] = 7;                 // OK, meaning \tcode{x.\keyword{operator}[](\{1,2,3\})}