Fix incorrect comment in lgc.h (#1288)

The comment gave an incorrect (reversed) version of the invariant, which
could be confusing for people who haven't read the full description in
lgc.cpp.

Unfortunately this change is difficult to flag.

Fixes #1282.

VM/src/lgc.h

@@ -23,11 +23,10 @@
 #define GCSsweep 4
 
 /*
-** macro to tell when main invariant (white objects cannot point to black
-** ones) must be kept. During a collection, the sweep
-** phase may break the invariant, as objects turned white may point to
-** still-black objects. The invariant is restored when sweep ends and
-** all objects are white again.
+** The main invariant of the garbage collector, while marking objects,
+** is that a black object can never point to a white one. This invariant
+** is not being enforced during a sweep phase, and is restored when sweep
+** ends.
 */
 #define keepinvariant(g) ((g)->gcstate == GCSpropagate || (g)->gcstate == GCSpropagateagain || (g)->gcstate == GCSatomic)