Update docs/user-guide/references.rst
@@ -17,7 +17,7 @@ Glossary
1D Analysis
1D analysis or spectral analysis where data are reduced to a simple 1D
geometry along the reconstructed energy axis. In Cherenkov astronomy,
- this is classically performed with a OFF background measurement [Piron2001]_.
+ this is classically performed with an OFF background measurement [Piron2001]_.
3D Analysis
3D analysis or cube analysis, where data are reduced to a 3D cube with
