[temp.constr.atomic] Rephrase to make the meaning of "the same

*expression*" more obvious.

Author: zygoloid

source/templates.tex

@@ -1619,7 +1619,7 @@
 Two atomic constraints, $e_1$ and $e_2$, are
 \indextext{identical!atomic constraints|see{atomic constraint, identical}}%
 \defnx{identical}{atomic constraint!identical}
-if they are formed from the same
+if they are formed from the same appearance of the same
 \grammarterm{expression}
 and if, given a hypothetical template $A$
 whose \grammarterm{template-parameter-list} consists of