OpenGL: fix the README links.

Markdown doesn't work this way.

src/MagnumExternal/OpenGL/GL/README.md

@@ -1,8 +1,9 @@
 OpenGL header and extension loader is generated using flextGL, get it at
-[](https://github.com/ginkgo/flextGL).
+https://github.com/ginkgo/flextGL.
 
-See [](extensions.txt) for requested version and a list of non-core extensions.
+See [extensions.txt](extensions.txt) for requested version and a list of
-Call `flextGLgen.py` in this directory with the following arguments:
+non-core extensions. Call `flextGLgen.py` in this directory with the following
+arguments:
 
     .../flextGLgen.py -D . -t . extensions.txt
 

src/MagnumExternal/OpenGL/GLES2/README.md

@@ -1,9 +1,9 @@
 OpenGL header and extension loader is generated using flextGL, get it at
 https://github.com/ginkgo/flextGL.
 
-See [extensions.txt](extensions.txt) for requested version and a list of non-core extensions.
+See [extensions.txt](extensions.txt) for requested version and a list of
-Call `flextGLgen.py` in this directory with the following arguments to generate
+non-core extensions. Call `flextGLgen.py` in this directory with the following
-files for generic GLES2 implementations:
+arguments to generate files for generic GLES2 implementations:
 
     .../flextGLgen.py -D . -t . extensions.txt
 

src/MagnumExternal/OpenGL/GLES3/README.md

@@ -1,8 +1,9 @@
 OpenGL header and extension loader is generated using flextGL, get it at
-[](https://github.com/ginkgo/flextGL).
+https://github.com/ginkgo/flextGL.
 
-See [](extensions.txt) for requested version and a list of non-core extensions.
+See [extensions.txt](extensions.txt) for requested version and a list of
-Call `flextGLgen.py` in this directory with the following arguments:
+non-core extensions. Call `flextGLgen.py` in this directory with the following
+arguments:
 
     .../flextGLgen.py -D . -t . extensions.txt