PyDocs: GPU Module: Examples: convert all gl_FragColor variables to FragColor

Based on D6425 by @robbott

Differential Revision: https://developer.blender.org/D6425

doc/python_api/examples/gpu.7.py

@@ -56,10 +56,11 @@ fragment_shader = '''
     uniform sampler2D image;
 
     in vec2 uvInterp;
+    out vec4 FragColor;
 
     void main()
     {
-        gl_FragColor = texture(image, uvInterp);
+        FragColor = texture(image, uvInterp);
     }
 '''