Passing Vector by reference C++

Question

I am passing a string vector by reference, but the changes don't persist to the original variable. If I read all the strings at the end of StringFormatter, they will be correctly formatted. However, if I print out the strings as shown in main(), it will not show any of the modifications.

void StringFormatter(std::vector<std::string> &instructions){
    for(auto it = instructions.begin(); it < instructions.end(); it++){     
        /*Remove any line which has character ;
         Logic here doesn't really pertain to my question
         Tons more formatting in here, but not relevant.*/

        size_t pos = (*it).find_first_of(";",0);
        if(pos != std::string::npos){
            instructions.erase(it);
        } 
    }
}

int main(){
    /* instructions is populated through reading a text file*/
    /* vector of instructions is populated for us */

    StringFormatter(instructions);
    /*I want the changes made to instructions in StringFormatter() to modify the 
     original instructions variable in main()*/


    for(auto it = instructions.begin(); it < instructions.end(); it++){
        printf("%s\n", (*it).c_str());
    } 

    return 1;
}