Renewal equation notation should be consistent between READMEs and tutorials

[cdc-mitzimorris]

The PyRenew package README.md and docs index.md files define the observation process model in terms of ($\mu(t)$, $\pi(s)$, $\alpha$), but the counts observation process tutorial uses notation ($\lambda_t$, $p_d$).

PyRenew/README.md

Line 17 in 3ba4ab0

$$\mu(t) = \alpha \sum_{s} I(t-s) \, \pi(s)$$

PyRenew/docs/tutorials/observation_processes_counts.qmd

Line 44 in 3ba4ab0

$$\lambda_t = \alpha \cdot \sum_{d=0}^{D} I_{t-d} \cdot p_d$$

change the tutorial to match the READMEs.