improc ass4: Fixed typo.

improc/ass4/report/report.tex

@@ -135,7 +135,7 @@ G = convolve(F, Gauss(s), mode='nearest')
 \end{verbatim}
 
 The timing results of the first code snippet are displayed in figure
-\ref{fig:times-1d}. The graphs shows that the 1D convolution has a
+\ref{fig:times-1d}. The graph shows that the 1D convolution has a
 computational complexity of $\mathcal{O}(\sigma)$, which is much faster than
 the 2D convolution (certainly for higher scales).