runtime: fix missing space in error message

Change-Id: I422708d50c3c727246e7991039877660ca034dc9
Reviewed-on: https://go-review.googlesource.com/30144
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -421,7 +421,7 @@ func heapBitsForObject(p, refBase, refOff uintptr) (base uintptr, hbits heapBits
 			} else {
 				print(" to unused region of span")
 			}
-			print("idx=", hex(idx), " span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", s.state, "\n")
+			print(" idx=", hex(idx), " span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", s.state, "\n")
 			if refBase != 0 {
 				print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
 				gcDumpObject("object", refBase, refOff)