[time.clock.system.overview] Remove superfluous whitespaces (#7815)

Author: Andreas-Krug

source/time.tex

@@ -2655,8 +2655,8 @@
     static time_point now() noexcept;
 
     // mapping to/from C type \tcode{time_t}
-    static time_t      to_time_t  (const time_point& t) noexcept;
-    static time_point  from_time_t(time_t t) noexcept;
+    static time_t     to_time_t  (const time_point& t) noexcept;
+    static time_point from_time_t(time_t t) noexcept;
   };
 }
 \end{codeblock}