remove default shared. less confusing for new users

Working-on-GNU-Linux.md

@@ -103,7 +103,7 @@ Building with cmake on Linux is easy, just use the following:
 git clone https://github.com/raysan5/raylib.git raylib
 cd raylib
 mkdir build && cd build
-cmake -DBUILD_SHARED_LIBS=ON ..
+cmake ..
 make
 sudo make install
 sudo ldconfig