[gdb/testsuite, gdbserver] Fix typos

Codespell noticed two new typos:
...
$ pre-commit run --all-files codespell
codespell................................................................Failed
- hook id: codespell
- exit code: 65

gdbserver/server.cc:4255: errror ==> error
gdb/testsuite/gdb.replay/missing-thread.exp:87: Whn ==> When
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Fix these.

gdbserver/server.cc

@@ -4252,7 +4252,7 @@ captured_main (int argc, char *argv[])
 	      optind--;
 
 	      /* For required arguments, if we don't have an argument, then
-		 this is an errror, set OPTC to reflect this.  */
+		 this is an error, set OPTC to reflect this.  */
 	      if (longopts[longindex].has_arg == required_argument)
 		optc = ':';
 	    }