Correct wording from previous PR (#14066)

# Description

Apologies - The updated wording I used in the last PR *description* was
not what I actually pushed. I failed to commit and push the last update.
This PR fixes the code to reflect what was described in #14065:

```
-r, --header-row - use the first input column as the table header-row (or keynames when combined with --as-record)
```

# User-Facing Changes

Help/doc only

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

(And visually confirmed help changes ;-))

# After Submitting

N/A

crates/nu-command/src/filters/transpose.rs

@@ -25,7 +25,7 @@ impl Command for Transpose {
             ])
             .switch(
                 "header-row",
-                "treat the first column as the header-row (column names)",
+                "use the first input column as the table header-row (or keynames when combined with --as-record)",
                 Some('r'),
             )
             .switch(