Typo in doc

docs/nstrophy_doc.tex

@@ -176,7 +176,7 @@ It can be made by specifying the parameter {\tt adaptive\_norm}.
   Another option is to define a norm based on the expression of the enstrophy\-~(\ref{enstrophy}):
   \begin{equation}
     \|f\|:=\frac1{\mathcal N}\sqrt{\sum_k k^2|f_k|^2}
-    \quad
+    ,\quad
     \mathcal N:=\left(\frac{\sqrt{\sum_k k^2|\hat u_k^{(n)}|^2}+\sqrt{\sum_k k^2|\hat U_k^{(n)}|^2}}{\sum_k k^2|\hat u_k^{(n)}|^2}\right)^{\frac13}
     .
   \end{equation}