GUI issue #54: Fix typo

Author: kmokstad

src/UsersGuide/mechanism_elements.tex

@@ -382,7 +382,7 @@
   you specify the axial stiffness ({\sl EA}),
   the bending stiffnesses ({\sl EIyy} and {\sl EIzz}),
   the torsional stiffness ({\sl GIt}),
-  the mass per unit length ($\rho${\sl L})
+  the mass per unit length ($\rho${\sl A})
   and the torsional moment of inertia ($\rho${\sl Ip}).
 
 \item{\sl Dependent properties} --