[diff.cpp23.expr] Remove duplicate "that" (#7134)

Author: Andreas-Krug

source/compatibility.tex

@@ -39,7 +39,7 @@
 \rationale
 Reduce undefined behavior.
 \effect
-A valid \CppXXIII{} program that that calls \tcode{delete} on an incomplete
+A valid \CppXXIII{} program that calls \tcode{delete} on an incomplete
 class type is ill-formed.
 \begin{example}
 \begin{codeblock}