Change the title of the "Set the region" and "Plot lines" tutorials (#1139)

Author: xdshivani

examples/tutorials/lines.py

@@ -1,6 +1,6 @@
 """
-Plot lines
-==========
+Plotting lines
+==============
 
 Plotting lines is handled by :meth:`pygmt.Figure.plot`.
 

examples/tutorials/regions.py

@@ -1,6 +1,6 @@
 """
-Set the region
-==============
+Setting the region
+==================
 
 Many of the plotting functions take the ``region`` parameter, which sets
 the area that will be shown in the figure. This tutorial covers the different types of