[rand.eng.philox] Replace repetitive note with reference to earlier text.

Author: tkoeppe

source/numerics.tex

@@ -3297,9 +3297,7 @@
 For $j = 0, \dotsc, n - 1$ sets $X_j$ to $C_{n - 1 - j} \mod 2^w$.
 The value of $i$ is set to $n - 1$.
 \begin{note}
-%FIXME: What is "Counter"?
-Counter is an unsigned integer value
-$\sum_{j = 0}^{n - 1} X_j \cdot 2^{wj}$ of $n \cdot w$ bits.
+The counter is the value $Z$ introduced at the beginning of this subclause.
 \end{note}
 \end{itemdescr}