[res.on.preconditions] Replace obsolete term 'expects' in the title.

Also rename the stable label from [res.on.expects], which was
introduced recently by commit 829b921.

Author: jensmaurer

source/lib-intro.tex

@@ -2976,7 +2976,7 @@
 This applies even to objects such as mutexes intended for thread synchronization.
 \end{note}
 
-\rSec3[res.on.expects]{Expects paragraph}
+\rSec3[res.on.preconditions]{Preconditions paragraph}
 
 \pnum
 Violation of any preconditions specified in a function's \expects element