[temp.deduct] Fixes from editorial review of CWG2293:

* Use "not valid" rather than "invalid", since only "valid" has been defined.
* Add an "otherwise" to improve flow.

Author: tkoeppe

source/templates.tex

@@ -7073,9 +7073,9 @@
 
 \pnum
 When an explicit template argument list is specified, if the
-given \grammarterm{template-id} is invalid\iref{temp.names},
+given \grammarterm{template-id} is not valid\iref{temp.names},
 type deduction fails.
-The specified template argument values are substituted for the
+Otherwise, the specified template argument values are substituted for the
 corresponding template parameters as specified below.
 
 \pnum