Fix docs generation for datetime with doxygen 1.11.0

It seems that as of the below commit, doxygen changed its handling of
parsing backticks in comments such that it now fails to properly
generate documentation for the entire datetime.h file.  Fix this by
closing the open backtick.

Also update the actual, non-documentation, header to match for
consistency, even if non-matching backticks are not a problem there.

See: https://github.com/doxygen/doxygen/commit/f18767307be20ca8d2ca81f74cc1f3446205282b

Closes #24814.

include/wx/datetime.h

@@ -149,7 +149,7 @@ public:
         Local,
 
         // zones from GMT (= Greenwich Mean Time): they're guaranteed to be
-        // consequent numbers, so writing something like `GMT0 + offset' is
+        // consequent numbers, so writing something like `GMT0 + offset` is
         // safe if abs(offset) <= 12
 
         // underscore stands for minus

interface/wx/datetime.h

@@ -96,7 +96,7 @@ public:
 
         ///@{
         /// zones from GMT (= Greenwich Mean Time): they're guaranteed to be
-        /// consequent numbers, so writing something like `GMT0 + offset' is
+        /// consequent numbers, so writing something like `GMT0 + offset` is
         /// safe if abs(offset) <= 12
 
         // underscore stands for minus