Update README.md: math syntax on Gitlab markdown is enabled by means of a...

Update README.md: math syntax on Gitlab markdown is enabled by means of a different sentinel (see readme file). An artifact was improperly parsed (&). Verify if formula is still correct 

README.md

@@ -2,7 +2,7 @@
 
 An oil & gas code had the openmp-critical-section pattern and the computational aspects of
 the original code is recreated here. This application solves the 3D wave equation:
-$$\frac{\partial^{2}u}{\partial t^{2}} & = c^{2}\nabla^{2}u$$
+$`\frac{\partial^{2}u}{\partial t^{2}} = c^{2}\nabla^{2}u`$
 using the pseudospectral method. However, for the WP7 kernel the finite difference method was
 selected for the purpose of simplicity which re-created the computational profile of the
 original code.