Add missing -o requirements.txt in uv pip compile example (#12308)

## Summary

`uv pip compile` doesn't produce `requirements.txt` without `-o
requirements.txt` option.

docs/concepts/resolution.md

@@ -273,11 +273,12 @@ For example, given the following `requirements.in` file:
 flask>=2.0.0
 ```
 
-Running `uv pip compile requirements.in` would produce the following `requirements.txt` file:
+Running `uv pip compile requirements.in -o requirements.txt` would produce the following
+`requirements.txt` file:
 
 ```python title="requirements.txt"
 # This file was autogenerated by uv via the following command:
-#    uv pip compile requirements.in
+#    uv pip compile requirements.in -o requirements.txt
 blinker==1.7.0
     # via flask
 click==8.1.7
@@ -295,11 +296,12 @@ werkzeug==3.0.1
     # via flask
 ```
 
-However, `uv pip compile --resolution lowest requirements.in` would instead produce:
+However, `uv pip compile --resolution lowest requirements.in -o requirements.txt` would instead
+produce:
 
-```python title="requirements.in"
+```python title="requirements.txt"
 # This file was autogenerated by uv via the following command:
-#    uv pip compile requirements.in --resolution lowest
+#    uv pip compile --resolution lowest requirements.in -o requirements.txt
 click==7.1.2
     # via flask
 flask==2.0.0