Add newline to check syntax output

[rok-cesnovar]

Summary

Running the following script:

library(cmdstanr)

file <- system.file("logistic.stan", package = "cmdstanr")

model <- cmdstan_model(stan_file=file, compile=FALSE)
model$check_syntax(stanc_options = list("debug-mem-patterns"))

produced the following output:

vector[K] beta: AoSStan program is syntactically correct

which is missing a newline. This PR fixes this issue. This was reported by @avehtari.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #689 (0cb44c6) into master (dd01ab1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #689      +/-   ##
==========================================
+ Coverage   89.95%   89.98%   +0.02%     
==========================================
  Files          12       12              
  Lines        3654     3653       -1     
==========================================
  Hits         3287     3287              
+ Misses        367      366       -1     

Impacted Files	Coverage Δ
R/model.R	91.42% <100.00%> (+0.13%)	⬆️

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