[FIX] Typo in reference_frame description (SpatialAxis -> SpatialAxes)

[yarikoptic]

Should be plural

❯ git grep SpatialAxes
src/modality-specific-files/motion.md:| SpatialAxes   | RECOMMENDED                                            | [string][] | The coordinate system in which the motion data are to be interpreted. A sequence of characters from the set `{'A', 'P', 'L', 'R', 'S', 'I', '_'}` indicating the direction of each axis. For example `"ARS"` indicates positive values in the X, Y, Z axes are respectively anterior, right, and superior of the origin, while `"PLI"` indicates positive values are posterior, left, and inferior of the origin. The `"_"` character may be used for unused axes. |
src/modality-specific-files/motion.md:        "SpatialAxes": "ALS",
src/schema/objects/columns.yaml:    `RotationOrder`, and `SpatialAxes`, and MAY use `Description` for the specification.
src/schema/objects/metadata.yaml:SpatialAxes:
src/schema/objects/metadata.yaml:  name: SpatialAxes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.93%. Comparing base (05f64ed) to head (ea2223f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1721 +/- ## ======================================= Coverage 87.93% 87.93% ======================================= Files 16 16 Lines 1351 1351 ======================================= Hits 1188 1188 Misses 163 163 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[effigies]

@yarikoptic Do you want this in the changelog? If not, please add the exclude-from-changelog label.