[allocator.requirements] Fix missing colon for "u" in Table 27 (#195)

Fixes #195.

Author: Dawn Perchik

source/lib-intro.tex

@@ -1680,7 +1680,7 @@
   conversion from a value \tcode{p}  \\ \rowsep
 \tcode{x}       &   a value of type \tcode{XX::const_void_pointer} obtained by
   conversion from a value \tcode{q} or a value \tcode{w}  \\ \rowsep
-\tcode{y}       &   a value of type \tcode{XX:const_void_pointer} obtained by
+\tcode{y}       &   a value of type \tcode{XX::const_void_pointer} obtained by
 conversion from a result value of \tcode{YY::allocate}, or else a value of
 type (possibly \tcode{const}) \tcode{std::nullptr_t}. \\ \rowsep
 \tcode{n}       &   a value of type \tcode{XX::size_type}.   \\ \rowsep