[bit.cast] Use colon to introduce list to avoid ambiguity.

During LWG discussion it was suggested that a colon makes it clearer
that the subsequent list applies only in the context of "u".

Author: tkoeppe

source/utilities.tex

@@ -18873,7 +18873,7 @@
 for which the smallest enclosing object has an erroneous value.
 For each bit $b$ in the value representation of the result
 that is indeterminate or erroneous,
-let $u$ be the smallest object containing that bit enclosing $b$.
+let $u$ be the smallest object containing that bit enclosing $b$:
 \begin{itemize}
 \item
 If $u$ is of unsigned ordinary character type or \tcode{std::byte} type,