FindOpenGL: Fix typo in documented target name

We provide "OpenGL::GL", not "OpenGL::OpenGL".

Reported-by: Daniel Pfeifer

Modules/FindOpenGL.cmake

@@ -9,7 +9,7 @@
 #
 # This module defines the :prop_tgt:`IMPORTED` targets:
 #
-# ``OpenGL::OpenGL``
+# ``OpenGL::GL``
 #  Defined if the system has OpenGL.
 # ``OpenGL::GLU``
 #  Defined if the system has GLU.