gtestutils: Fix section marker in documentation

Apparently 3 hashes is too many, and as a result this line is not
displayed properly in the HTML documentation. 2 hashes seems to be
just right, as can be seen with:

  $ git grep '^ \* ##' | wc -l
  65
  $ git grep '^ \* ###' | wc -l
  1

glib/gtestutils.c

@@ -174,7 +174,7 @@
  * }
  * ]|
  *
- * ### Integrating GTest in your project
+ * ## Integrating GTest in your project
  *
  * If you are using the [Meson](http://mesonbuild.com) build system, you will
  * typically use the provided `test()` primitive to call the test binaries,