[temp.class.spec] Fix index entry for 'primary class template'. (#1446)

Fixes #1441.

Author: jensmaurer

source/templates.tex

@@ -2181,9 +2181,9 @@
 
 \pnum
 \indextext{specialization!class template partial}%
-\indextext{template!primary}%
+\indextext{primary class template|see{template, primary}}%
 A
-\defnx{primary class template}{see{template, primary}}
+\defnx{primary class template}{template!primary}
 declaration is one in which the class template name is an
 identifier.
 A template declaration in which the class template name is a