[examples][shallowwater][main] remove ()

examples/shallowwaterfriction/doc/main_intro.md

@@ -3,10 +3,9 @@
 We want to solve a shallow water flow problem in a rough
 channel to obtain the water table and
 compare it to the analytic solution. This is done with the
-`main()` function
+`main` function
 of the program which is defined in the file `main.cc` described below.
 
 The code documentation is structured as follows:
 
 [[_TOC_]]
-