fix typo

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12211 2fb0f335-1f38-0410-981e-8018bf24f1b0

doc/handbook/guidelines.tex

@@ -31,7 +31,7 @@ The only exception: chemical formulas are written in their chemically correct wa
 \item Variables should be named as self-explaining as possible: especially abbreviations 
 should be avoided (saturation in stead of S)
 \item Method parameters which are not self-explanatory should always
-  have a meaningful comment a at calling sites. Example:
+  have a meaningful comment at calling sites. Example:
 \begin{lstlisting}[style=DumuxCode]
    localResidual.eval(/*includeBoundaries=*/true);
 \end{lstlisting}