Update README.md: Mention uvx is an alias for uv tool run (#6286)

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## Summary

I think it's important to clarify that `uvx` is simply an alias for `uv
tool run`. This distinction helps avoid confusion about when to use `uv`
versus `uvx`. I thought the [blog
post](https://astral.sh/blog/uv-unified-python-packaging) explained this
well.

Just something that I ran into, I understand others may have a different
perspective!

## Test Plan

n/a

---------

Co-authored-by: Zanie Blue <[email protected]>

README.md

@@ -99,7 +99,7 @@ See the [project documentation](https://docs.astral.sh/uv/guides/projects/) to g
 
 uv executes and installs command-line tools provided by Python packages, similar to `pipx`.
 
-Run a tool in an ephemeral environment with `uvx`:
+Run a tool in an ephemeral environment using `uvx` (an alias for `uv tool run`):
 
 ```console
 $ uvx pycowsay 'hello world!'

docs/index.md

@@ -83,7 +83,7 @@ See the [project guide](./guides/projects.md) to get started.
 
 uv executes and installs command-line tools provided by Python packages, similar to `pipx`.
 
-Run a tool in an ephemeral environment with `uvx`:
+Run a tool in an ephemeral environment using `uvx` (an alias for `uv tool run`):
 
 ```console
 $ uvx pycowsay 'hello world!'