replaced phase difference figure with better suited one, fixed units too

problemexpres.tex

@@ -70,7 +70,7 @@ Phasor is a vector that represents a sinusoidally varying quantity, as a current
 
 \begin{figure}[ht!]
 \centering
-\includegraphics[width=.75\textwidth,angle=0]{phase_difference}
+\includegraphics[width=.75\textwidth,angle=0]{phase_diff}
 \caption{The phase difference between voltage (blue) and current (red), the origin of phase difference of angle $\varphi$}\label{f:ph_diff}
 \end{figure}