[DOC] Fix Process::exec documentation [ci skip]

Shell reserved words and special built-in commands can be placed
at only the beginning of the command (except for leading white
spaces).

process.c

@@ -3155,7 +3155,7 @@ NORETURN(static VALUE f_exec(int c, const VALUE *a, VALUE _));
  *  these simple rules:
  *
  *  * no meta characters
- *  * no shell reserved word and no special built-in
+ *  * not starting with shell reserved word or special built-in
  *  * Ruby invokes the command directly without shell
  *
  *  You can force shell invocation by adding ";" to the string (because ";" is