[concept.booleantestable] Use tie before cross references

Author: opensdh

source/concepts.tex

@@ -846,7 +846,7 @@
 The exposition-only \exposconcept{boolean-testable} concept
 specifies the requirements on expressions
 that are convertible to \tcode{bool} and
-for which the logical operators
+for which the logical operators~%
 (\ref{expr.log.and}, \ref{expr.log.or}, \ref{expr.unary.op})
 have the conventional semantics.