chore: move 'job' to experimental category (#15568)

# Description

The 'job' command was incorrectly placed into the "Strings" category
rather than the "Experimental" category like its subcommands. This PR
resolves that issues.

# User-Facing Changes

Changes to where the `job` command is found when using the `help`
command or reading the documentation.

crates/nu-command/src/experimental/job.rs

@@ -10,7 +10,7 @@ impl Command for Job {
 
     fn signature(&self) -> Signature {
         Signature::build("job")
-            .category(Category::Strings)
+            .category(Category::Experimental)
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }