doc: Install [dev] so that docs can be built

The 'make html' command fails witht he [doc] build. Update the
documentation to show how to resolve this.

Fixes: labgrid-project#1352

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/development.rst

@@ -679,7 +679,7 @@ When contributing to documentation it's practical to be able to build it also lo
 
     git clone https://github.com/labgrid-project/labgrid.git
     cd labgrid
-    pip install -e ".[doc]"
+    pip install -e ".[dev]"
     cd doc
     make html