fix `WordOccurrence` algorithm

From the output of [checkWordOccurrence](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckWordOccurrence.js), we could see it's should be "**Letter**" occurrence instead of "**Word**".

[Test Case](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CheckWordOcurrence.test.js)