Fix `is_continuous()` bug for normal distributions

[joshwlambert]

The PR addresses #421 by correcting a typo in the character string matching in is_continuous(). This bug was leading to is_continuous() incorrectly returning FALSE for normal distributions in <epiparameter> objects. This impacted the <epiparameter> plot method, see reprex in #421.

This PR resolves the issue:

library(epiparameter)
db <- epiparameter_db()
#> Returning 125 results that match the criteria (100 are parameterised). 
#> Use subset to filter by entry variables or single_epiparameter to return a single entry. 
#> To retrieve the citation for each use the 'get_citation' function
fam <- sapply(db, family)
norm_idx <- which(fam == "norm")
norm_ep <- db[norm_idx][[1]]
norm_ep
#> Disease: COVID-19
#> Pathogen: SARS-CoV-2
#> Epi Parameter: serial interval
#> Study: Yang L, Dai J, Zhao J, Wang Y, Deng P, Wang J (2020). "Estimation of
#> incubation period and serial interval of COVID-19: analysis of 178
#> cases and 131 transmission chains in Hubei province, China."
#> _Epidemiology and Infection_. doi:10.1017/S0950268820001338
#> <https://doi.org/10.1017/S0950268820001338>.
#> Distribution: norm (days)
#> Parameters:
#>   mean: 4.600
#>   sd: 4.400
norm_ep$prob_distribution
#> <distribution[1]>
#> [1] N(4.6, 19)
is_continuous(norm_ep)
#> [1] TRUE
plot(norm_ep)

^{Created on 2024-11-27 with reprex v2.1.0}