remove mention of __fish_sgrep

This is a trivial change to address issue #2450 by eliminating a
inconsequential reference to `__fish_sgrep` in the core C++ code.

src/complete.cpp

@@ -1567,8 +1567,7 @@ static wrapper_map_t &wrap_map() {
     return *wrapper_map;
 }
 
-/// Add a new target that is wrapped by command. Example: __fish_sgrep (command) wraps grep
-/// (target).
+/// Add a new target that wraps a command. Example: __fish_XYZ (function) wraps XYZ (target).
 bool complete_add_wrapper(const wcstring &command, const wcstring &new_target) {
     if (command.empty() || new_target.empty()) {
         return false;