[exec.util.cmplsig.trans] Fix English grammar (cplusplus#7188)

Author: Andreas-Krug

source/exec.tex

@@ -4849,7 +4849,7 @@
 adapt the completion signatures of \tcode{Sndr} by
 lvalue-ref qualifying the values,
 adding an additional \tcode{exception_ptr} error completion
-if its not already there, and
+if it is not already there, and
 leaving the other completion signatures alone.
 \begin{codeblock}
 template<class... Args>