ShaderUI : Fix user defaults for parameter child plugs

Fixes the following test failure :

```
======================================================================
FAIL: testSplineParameterUserDefaults (GafferOSLUITest.OSLShaderUITest.OSLShaderUITest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/john/dev/build/gaffer/python/GafferOSLUITest/OSLShaderUITest.py", line 110, in testSplineParameterUserDefaults
    self.assertEqual( s["parameters"]["spline"]["interpolation"].getValue(), Gaffer.SplineDefinitionInterpolation.Linear )
AssertionError: 1 != Gaffer._Gaffer.SplineDefinitionInterpolation.Linear
```

This appears to have been broken by GafferHQ#2536, which was pull-requested before the test was added, but merged after the test had been merged. Something to bear in mind if we keep PRs open for a long time - the Travis results are only valid at the point they were run, and aren't to be trusted after subsequent changes to master.

python/GafferSceneUI/ShaderUI.py

@@ -121,7 +121,7 @@ def __parameterUserDefault( plug ) :
 
 		],
 
-		"parameters.*" : [
+		"parameters..." : [
 
 			# Although the parameters plug is positioned
 			# as we want above, we must also register