Update CONTRIBUTING.md

Changed the appearance of the written directory to match documentation

Author: alanbu1

CONTRIBUTING.md

@@ -42,7 +42,7 @@ If you are considering larger changes its best to discuss them first (e.g. file
 Once you've made your enhancements [open a pull request](#making-pull-requests) to get your changes considered for inclusion in the documentation.
 
 ### How do I build the documentation?
-The documentation can be built by using the command `make html` in the `$VTR_ROOT/doc` directory and you can view it in a web browser by loading the file at $VTR_ROOT/_build/html/index.html. More information on building
+The documentation can be built by using the command `make html` in the `$VTR_ROOT/doc` directory and you can view it in a web browser by loading the file at `$VTR_ROOT/_build/html/index.html`. More information on building
 the documentation can be found on the [README on GitHub](https://github.com/verilog-to-routing/vtr-verilog-to-routing/tree/master/doc).
 
 ### Filling Bug Reports