You cannot reset the light without a scan tool. However, if the cap was the source of your problem, the computer will shut off the light next time it performs an EVAP system test and finds that it passes. The EVAP system works by absorbing gas vapors in a tank of activated charcoal. When certain driving conditions are met, the computer sends a command to open both a purge and a vent solenoid that allows the engine to suck fresh air through the canister and "purge" it of gas vapors. The engine burns the vapors. When it completes the purge, it tests the system to see if it holds vacuum. If it doesn't, it lights your check engine light to notify you of a leak.