Fixes bug in outlier plot method

[rempsyc]

Closes #711

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# Prepare model
library(see)
library(lme4)
library(performance)
set.seed(123)  # for reproducibility
n <- 100
subjectID <- paste("Subject", rep(1:10, each = 10))
PN <- runif(n, 0, 100)
alpha <- 0.5 * PN + rnorm(n, mean = 50, sd = 10)
alpha[c(20, 40)] <- c(150, 160)  # arbitrarily chosen subjects for outliers
data <- data.frame(subjectID, PN, alpha)
model <- lmer(alpha ~ PN + (1 | subjectID), data = data)

# Test
x <- check_outliers(model)
plot(x)

^{Created on 2024-04-28 with reprex v2.1.0}