Rename Autofix to Fix (#7657)

**Summary** Mostly mechanical symbol rename and search-and-replace, with
small changes to the markdown docs to read better

crates/ruff_cli/src/commands/check_stdin.rs

@@ -16,7 +16,7 @@ pub(crate) fn check_stdin(
     pyproject_config: &PyprojectConfig,
     overrides: &CliOverrides,
     noqa: flags::Noqa,
-    autofix: flags::FixMode,
+    fix_mode: flags::FixMode,
 ) -> Result<Diagnostics> {
     if let Some(filename) = filename {
         if !python_file_at_path(filename, pyproject_config, overrides)? {
@@ -33,7 +33,7 @@ pub(crate) fn check_stdin(
         stdin,
         &pyproject_config.settings,
         noqa,
-        autofix,
+        fix_mode,
     )?;
     diagnostics.messages.sort_unstable();
     Ok(diagnostics)