How can i build a machine learning model to identify a specific word?
It is always best to approach a problem using the simplest possible tools. In your case there is no need to use a machine learning (AI) model to detect the word sale. 

To find the word sale within a long datastream, I would do the following: parse through the entire String while looking at segments of size 4 at a time, convert the substring to lowercase. Determine if this substring is "sale". If it is save the index of the "s" into a list. Return the list which will include the index of each start of the word sale within your text.

public ArrayList&lt;Integer&gt; getIndexSale(String inputString){
   ArrayList&lt;Integer&gt; indexes = new ArrayList&lt;&gt;();
   String[] queue = new String[4];

   queue[0] = 0;
   queue[1] = 0;
   queue[2] = 0;
   queue[3] = 0;

   for(int i = 0; i&lt;inputString.length(); i++){
      queue[0] = queue[1];
      queue[1] = queue[2];
      queue[2] = queue[3];
      queue[3] = inputString.charAt(i);

      String temp = java.util.Arrays.toString(queue).toLowerCase();

      if(temp.equals("sale"))  indexes.add(i);
   }

   return indexes;
} 


As mentioned by Dan Carter: if you do not care about the indices of the word "sale" within your string then it is simpler to use:

public boolean getIndexSale(String inputString){

   if(inputString.toLowerCase().contains("sale")) return true;
   return false;
}