Fix Jupyter Lite serve instruction

Author: mcbarton

Emscripten-build-instructions.md

@@ -352,5 +352,7 @@ jupyter lite build --XeusAddon.prefix=$PREFIX \
 Once the Jupyter Lite site has built you can test the website locally by executing
 
 ```bash
-jupyter lite serve --XeusAddon.prefix=$PREFIX
+jupyter lite serve --XeusAddon.prefix=$PREFIX \
+                   --XeusAddon.mounts="$PREFIX/share/xeus-cpp/tagfiles:/share/xeus-cpp/tagfiles" \
+                   --XeusAddon.mounts="$PREFIX/etc/xeus-cpp/tags.d:/etc/xeus-cpp/tags.d"
 ```

docs/Emscripten-build-instructions.rst

@@ -376,4 +376,6 @@ executing
 
 .. code:: bash
 
-   jupyter lite serve --XeusAddon.prefix=$PREFIX
+   jupyter lite serve --XeusAddon.prefix=$PREFIX \
+                      --XeusAddon.mounts="$PREFIX/share/xeus-cpp/tagfiles:/share/xeus-cpp/tagfiles" \
+                      --XeusAddon.mounts="$PREFIX/etc/xeus-cpp/tags.d:/etc/xeus-cpp/tags.d"