Update documentation to address CRAN note on lost braces

[villegar]

Version: 4.2.0
Check: Rd files
Result: NOTE 
  checkRd: (-1) simpopmlx.Rd:21: Lost braces; missing escapes or markup?
      21 | \item{fim}{the Fisher Information Matrix estimated by Monolix. fim={"sa", "lin"} (default="sa")}
         |                                                                    ^
Flavors: [r-devel-linux-x86_64-debian-clang](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-debian-clang/mlxR-00check.html), [r-devel-linux-x86_64-debian-gcc](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-debian-gcc/mlxR-00check.html), [r-devel-linux-x86_64-fedora-clang](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-clang/mlxR-00check.html), [r-devel-linux-x86_64-fedora-gcc](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-gcc/mlxR-00check.html), [r-devel-windows-x86_64](https://www.r-project.org/nosvn/R.check/r-devel-windows-x86_64/mlxR-00check.html), [r-patched-linux-x86_64](https://www.r-project.org/nosvn/R.check/r-patched-linux-x86_64/mlxR-00check.html), [r-release-linux-x86_64](https://www.r-project.org/nosvn/R.check/r-release-linux-x86_64/mlxR-00check.html), [r-release-macos-arm64](https://www.r-project.org/nosvn/R.check/r-release-macos-arm64/mlxR-00check.html), [r-release-macos-x86_64](https://www.r-project.org/nosvn/R.check/r-release-macos-x86_64/mlxR-00check.html), [r-release-windows-x86_64](https://www.r-project.org/nosvn/R.check/r-release-windows-x86_64/mlxR-00check.html), [r-oldrel-macos-arm64](https://www.r-project.org/nosvn/R.check/r-oldrel-macos-arm64/mlxR-00check.html), [r-oldrel-macos-x86_64](https://www.r-project.org/nosvn/R.check/r-oldrel-macos-x86_64/mlxR-00check.html), [r-oldrel-windows-x86_64](https://www.r-project.org/nosvn/R.check/r-oldrel-windows-x86_64/mlxR-00check.html)