[semaphore.syn] Fix typeface for "implementation-defined" text.

This is implementation-defined code, so it should be italic code font
not italic Roman font.

Author: zygoloid

source/threads.tex

@@ -5097,7 +5097,7 @@
 \indexheader{semaphore}%
 \begin{codeblock}
 namespace std {
-  template<ptrdiff_t least_max_value = @\textrm{\textit{implementation-defined}}@>
+  template<ptrdiff_t least_max_value = @\impdef@>
     class counting_semaphore;
 
   using binary_semaphore = counting_semaphore<1>;
@@ -5108,7 +5108,7 @@
 
 \begin{codeblock}
 namespace std {
-  template<ptrdiff_t least_max_value = @\textrm{\textit{\impldef{value for \tcode{least_max_value} default template argument of \tcode{counting_semaphore}}}}@>
+  template<ptrdiff_t least_max_value = @\textit{\impldef{value for \tcode{least_max_value} default template argument of \tcode{counting_semaphore}}}@>
   class counting_semaphore {
   public:
     static constexpr ptrdiff_t max() noexcept;