fix: use --override rather than -o to specify overrides in README.md (#…

…1668)

## Summary

Fix documentation (Readme.md):
`-o` is the short for `--output-file`, so using `-o overrides.txt` would
store the output in `overrides.txt` rather than using that as overrides.

## Test Plan

no tests for docs (yet?)

README.md

@@ -275,7 +275,7 @@ Historically, `pip` has supported "constraints" (`-c constraints.txt`), which al
 narrow the set of acceptable versions for a given package.
 
 uv supports constraints, but also takes this concept further by allowing users to _override_ the
-acceptable versions of a package across the dependency tree via overrides (`-o overrides.txt`).
+acceptable versions of a package across the dependency tree via overrides (`--override overrides.txt`).
 
 In short, overrides allow the user to lie to the resolver by overriding the declared dependencies
 of a package. Overrides are a useful last resort for cases in which the user knows that a