[examples][tracer] Slighty adjust wording in main intro doc

examples/1ptracer/doc/main.md

@@ -6,12 +6,14 @@
 
 # Part 3: Main program flow
 
-This file contains the main program flow. In this example, we solve a single-phase flow problem
-to obtain a pressure distribution in the domain. Subsequently, the distribution of volume fluxes
-is computed from that pressure distribution, which is then passed to a tracer problem to solve
-the transport of an initial contamination through the model domain.
-
-The subsequent code documentation is structured as follows:
+We want to solve a single-phase flow problem
+to obtain a pressure distribution in the domain. Subsequently, we compute a volume fluxes field
+based on the obtained pressure distribution and pass it to the tracer problem.
+Finally, we solve a transient transport problem for a tracer using the computed volume fluxes.
+This main program flow is implemented in the `main()` function
+of the program which is defined in the file `main.cc` described below.
+
+The code documentation is structured as follows:
 
 [[_TOC_]]
 

examples/1ptracer/doc/mainintro.md

 # Part 3: Main program flow
 
-This file contains the main program flow. In this example, we solve a single-phase flow problem
-to obtain a pressure distribution in the domain. Subsequently, the distribution of volume fluxes
-is computed from that pressure distribution, which is then passed to a tracer problem to solve
-the transport of an initial contamination through the model domain.
+We want to solve a single-phase flow problem
+to obtain a pressure distribution in the domain. Subsequently, we compute a volume fluxes field
+based on the obtained pressure distribution and pass it to the tracer problem.
+Finally, we solve a transient transport problem for a tracer using the computed volume fluxes.
+This main program flow is implemented in the `main()` function
+of the program which is defined in the file `main.cc` described below.
 
-The subsequent code documentation is structured as follows:
+The code documentation is structured as follows:
 
 [[_TOC_]]