primsrast.txt: fix variable name. fixes flutter#8

Author: Vtec234

doc/specs/vulkan/chapters/primsrast.txt

@@ -87,7 +87,7 @@ as follows:
     is generated based on the value of the alpha component of the fragment's
     first color output as specified in the <<fragops-covg,Multisample
     Coverage>> section.
-  * pname:alphaToOne controls whether the value of the alpha component of
+  * pname:alphaToOneEnable controls whether the value of the alpha component of
     the fragment's first color output is replaced with one as described in
     <<fragops-covg,Multisample Coverage>>.