replace broken RTools43 link

previous link lead to an error

src/cmdstan-guide/installation.qmd

@@ -458,7 +458,7 @@ RTools43 provides g++-12. The installation/configuration is identical for both t
 Download the installer for your preferred toolchain and complete the prompts for installation:
 
  - [RTools42](https://cran.r-project.org/bin/windows/Rtools/rtools42/files/rtools42-5355-5357.exe)
- - [RTools43](https://cran.r-project.org/bin/windows/Rtools/rtools43/files/rtools43-5863-5818.exe)
+ - [RTools43](https://cran.r-project.org/bin/windows/Rtools/rtools43/files/rtools43-5958-5975.exe)
 
 Next, you need to add the toolchain directory to your `PATH` variable. Add the appropriate lines
 from below: