docs(dev_tools): fix how to properly run not yet installed build

Problem: running ./build/bin/nvim without make install
- doesn't respect local changes in ./runtime,
- includes the path where Nvim would be installed,
- ignores changes in precompiled Lua modules (like .../vim/_editor.lua)

Solution:
- use VIMRUNTIME=./runtime,
- use --luamod-dev

runtime/doc/dev_tools.txt

@@ -21,7 +21,7 @@ You can start hacking on Nvim in less than 5 minutes:
         cd neovim
         # (Optional) Build and run Nvim:
         make
-        ./build/bin/nvim
+        VIMRUNTIME=./runtime ./build/bin/nvim --luamod-dev
 3. Run a single test. We will start with "example_spec.lua", which is a real
    test that shows how tests are written: >
         make functionaltest TEST_FILE=test/functional/example_spec.lua