Correct an example

Seems to have been broken since the v3 update!

siunitx.tex

@@ -2730,11 +2730,11 @@ \subsection{Quantities}
 cases (very long units, narrow columns, \foreign{etc}.) where breaks may be
 needed. This can be turned on using the \opt{allow-quantity-breaks} option.
 \begin{LaTeXdemo}
-  \begin{minipage}{2.55cm}
+  \begin{minipage}{3cm}
     % Gives an underfull hbox
-    Some filler text \qty{10}{\metre} \\
+    X\hspace{2.4cm}\qty{10}{\metre} \\
     \sisetup{allow-quantity-breaks}
-    Some filler text \qty{10}{\metre}
+    X\hspace{2.4cm}\qty{10}{\metre}
   \end{minipage}
 \end{LaTeXdemo}