[gdb/tdep] Fix typo in debug message

In microblaze_analyze_prologue in gdb/microblaze-tdep.c I came across:
...
	  microblaze_debug ("got addi r1,r1,%d; contnuing\n", imm);
...

Fix this by using "continuing".

Reviewed-By: Tom Tromey <tom@tromey.com>