The JSDoc comment states the valid position range is '1 to 75', but the code validates against topMaxValidPosition = 70.
This PR fixes the documentation to match the implementation.

String.prototype.substr() is deprecated and may be removed in future
JavaScript engine versions (Annex B of the ECMAScript specification).
Replaced all occurrences in the codebase with String.prototype.substring():
- src/algorithms/string/rabin-karp/rabinKarp.js
- src/algorithms/cryptography/polynomial-hash/__test__/SimplePolynomialHash.test.js
- src/algorithms/cryptography/polynomial-hash/__test__/PolynomialHash.test.js
The behavior is identical since all substr(start, length) calls are
converted to substring(start, start + length) with the same arguments.
All existing tests pass.