Minor spelling fixes in code comments, docs and output

This commit does not touch any variable names or any other actual code,
and thus should not in any way affect how the code works.

include/mysql/service_thd_rnd.h

@@ -18,7 +18,7 @@
   @file
   This service provides access to the thd-local random number generator.
 
-  It's preferrable over the global one, because concurrent threads
+  It's preferable over the global one, because concurrent threads
   can generate random numbers without fighting each other over the access
   to the shared rnd state.
 */