Change 'are expected to' -> 'should'

Ad discussed with @bilderbuchi.

chapters/unitexpressions.tex

@@ -94,7 +94,7 @@ \section{The Syntax of Unit Expressions}\label{the-syntax-of-unit-expressions}
 \section{Unit Checking}\label{unit-checking}
 
 How to verify that units are used in compatible ways is current not fully determined by the specification.
-This section gives rules for certain situations, but in general tools are expected to reason according to standard dimensional analysis.
+This section gives rules for certain situations, but in general tools should reason according to standard dimensional analysis.
 
 
 \subsection{Standard Dimensional Analysis}\label{standard-dimensional-analysis}