doc: Update documentation generation example in developer-notes.md

To correspond to the documentation style of e.g. src/test/README.md

Co-authored-by: pablomartin4btc <pablomartin4btc@gmail.com>

doc/developer-notes.md

@@ -338,9 +338,10 @@ Recommendations:
 
 ### Generating Documentation
 
-The documentation can be generated with `cmake --build <build_dir> --target docs`.
-The resulting files are located in `<build_dir>/doc/doxygen/html`; open
-`index.html` in that directory to view the homepage.
+Assuming the build directory is named `build`,
+the documentation can be generated with `cmake --build build --target docs`.
+The resulting files will be located in `build/doc/doxygen/html`;
+open `index.html` in that directory to view the homepage.
 
 Before building the `docs` target, you'll need to install these dependencies: