Update material_specification.adoc

Removed stray left bracket.

src/docs/asciidoc/jme3/advanced/material_specification.adoc

@@ -262,7 +262,6 @@ a|Textures/MyTex.jpg
 
 |===
 
-}
 
 The formatting of the value, depends on the type of the value that was specified in the J3MD file being extended. Examples are provided for every parameter type: