docs: fix generate_session argument naming in examples

[Adithya-bs]

Fixes #216

Overview

This PR fixes a naming discrepancy in the examples/ directory. The generate_session method was being called with the incorrect keyword argument secret=, which results in a TypeError.

In the actual library implementation (kiteconnect/connect.py), the parameter is defined as api_secret.

Changes

The argument name has been updated from secret to api_secret in the following example scripts to ensure they are functional for users:

• examples/simple.py
• examples/gtt_order.py
• examples/order_margins.py

Verification

Checked the method signature in connect.py:
def generate_session(self, request_token, api_secret):

The examples now correctly align with the library's method signature.