[basic.contract.eval] Insert missing space between words (#7748)

Author: Andreas-Krug

source/basic.tex

@@ -7376,7 +7376,7 @@
 \end{note}
 
 \pnum
-There is an observablecheckpoint\iref{intro.abstract} $C$
+There is an observable checkpoint\iref{intro.abstract} $C$
 that happens before $A$
 such that any other operation $O$
 that happens before $A$