updateQthirdState: Update Q-values at Third State
In jdtrat/dynaq: Tools to Simulate DynaQ Reinforcement Learning Algorithms

This function takes the Q$thirdStateVec dataframe initialized in the background and filters it to pull the Q-table index that matches the terminal state. It then updates the Q-value according to the Q-learning algorithm.

1	updateQthirdState(state, reward, alpha, gamma)

`state`	The state the agent is currently in.
`reward`	The reward an agent receives at the current state.
`alpha`	The learning rate alpha.
`gamma`	The temporal discounting factor gamma.