Update pico2_tutorial.md with pte generation explicitly (#15450)

Be explicit about the steps on how to generate
balanced_tiny_mlp_mnist.pte

### Summary
[PLEASE REMOVE] See [CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

[PLEASE REMOVE] If this PR introduces a fix or feature that should be
the upcoming release notes, please add a "Release notes: <area>" label.
For a list of available release notes labels, check out
[CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Author: psiddh

docs/source/pico2_tutorial.md

@@ -35,9 +35,10 @@ python export_mlp_mnist.py # Creates balanced_tiny_mlp_mnist.pte
 ## Step 2: Build Firmware for Pico2
 
 ```bash
-# Generate model
-
-python export_mlp_mnist.py # Creates balanced_tiny_mlp_mnist.pte
+# Generate model (Creates balanced_tiny_mlp_mnist.pte)
+cd ./examples/raspberry_pi/pico2
+python export_mlp_mnist.py
+cd -
 
 # Build Pico2 firmware (one command!)