Fix documentation for python platform (#3220)

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## Summary

The option `--platform` for `uv pip compile` was added in #3111 and was
later renamed to `--python-platform` #3146.
This PR updates the documentation such that the listed option is working
again.

## Test Plan

```bash
❯ uv --version
uv 0.1.37 (645d039 2024-04-23)
```

Before:
```bash
❯ uv pip compile requirements.in --platform=linux
error: unexpected argument '--platform' found

  tip: to pass '--platform' as a value, use '-- --platform'

Usage: uv pip compile <SRC_FILE>...

For more information, try '--help'.
```

After:
```bash
❯ uv pip compile requirements.in --python-platform=linux
Resolved 1 package in 215ms
# This file was autogenerated by uv via the following command:
#    uv pip compile requirements.in --python-platform=linux
uv==0.1.37
```

README.md

@@ -391,24 +391,24 @@ the current platform and Python version. Unlike Poetry and PDM, uv does not yet
 machine-agnostic lockfile ([#2679](https://github.com/astral-sh/uv/issues/2679)).
 
 However, uv _does_ support resolving for alternate platforms and Python versions via the
-`--platform` and `--python-version` command line arguments.
+`--python-platform` and `--python-version` command line arguments.
 
 For example, if you're running uv on macOS, but want to resolve for Linux, you can run
-`uv pip compile --platform=linux requirements.in` to produce a `manylinux2014`-compatible
+`uv pip compile --python-platform=linux requirements.in` to produce a `manylinux2014`-compatible
 resolution.
 
 Similarly, if you're running uv on Python 3.9, but want to resolve for Python 3.8, you can run
 `uv pip compile --python-version=3.8 requirements.in` to produce a Python 3.8-compatible resolution.
 
-The `--platform` and `--python-version` arguments can be combined to produce a resolution for
+The `--python-platform` and `--python-version` arguments can be combined to produce a resolution for
 a specific platform and Python version, enabling users to generate multiple lockfiles for
 different environments from a single machine.
 
 _N.B. Python's environment markers expose far more information about the current machine
-than can be expressed by a simple `--platform` argument. For example, the `platform_version` marker
+than can be expressed by a simple `--python-platform` argument. For example, the `platform_version` marker
 on macOS includes the time at which the kernel was built, which can (in theory) be encoded in
 package requirements. uv's resolver makes a best-effort attempt to generate a resolution that is
-compatible with any machine running on the target `--platform`, which should be sufficient for
+compatible with any machine running on the target `--python-platform`, which should be sufficient for
 most use cases, but may lose fidelity for complex package and platform combinations._
 
 ### Time-restricted reproducible resolutions