[basic.def.odr] Change "is required" to "shall"

Author: sdkrystian

source/basic.tex

@@ -514,7 +514,7 @@
 
 \pnum
 \indextext{type!incomplete}%
-A definition of a class is required to be reachable in every context in which
+A definition of a class shall be reachable in every context in which
 the class is used in a way that requires the class type to be complete.
 \begin{example}
 The following complete translation unit is well-formed,