Update sample definition in README

Clarify the definition of 'one sample' in the context of Uncertaintyx.

Author: heptaflar

README.md

@@ -86,8 +86,8 @@ core.
 > [!NOTE]
 > The batch dimension $M$ enumerates independent samples (e.g.,
 > sensor scans, simulations, ensemble members) but you get to define
-> what “one sample” is: a single pixel, a spectrum, a scan line, or
-> a spatiotemporal cubelet. Uncertaintyx treats that single sample as
+> what “one sample” is: a single pixel value, a spectrum, a scan line,
+> or a spatiotemporal cubelet. Uncertaintyx treats that single sample as
 > a tensor $x$, and the framework scales it to a batch $X$ of $M$ such
 > samples. Many remote‑sensing workflows implicitly assume “one sample
 > is one pixel”, but this is often an oversimplification that obscures