Minor word tweaking for vxrm and vxsat cc

Current wording might confused that is referring to whole vcsr, but
actually we only refer to those fields.

Author: kito-cheng

riscv-cc.adoc

@@ -73,8 +73,8 @@ Vector registers are not used for passing arguments or return values; we
 intend to define a new calling convention variant to allow that as a future
 software optimization.
 
-The `vxrm` and `vxsat` fields of `vcsr` are not preserved across calls and the
-value is unspecified upon entry.
+The `vxrm` and `vxsat` fields of `vcsr` are not preserved across calls and their
+values are unspecified upon entry.
 
 Procedures may assume that `vstart` is zero upon entry. Procedures may assume
 that `vstart` is zero upon return from a procedure call.