Fix the speeling of "purgeable" in Gr code

src/lazy/SkDiscardablePixelRef.cpp

Index: src/lazy/SkDiscardablePixelRef.cpp
diff --git a/src/lazy/SkDiscardablePixelRef.cpp b/src/lazy/SkDiscardablePixelRef.cpp
index 4b7d47b94d2bb4068f0dfcfbe66b3f752a294b88..b51daa6faa7ded8aa34649ef7e381662a2c76d81 100644
--- a/src/lazy/SkDiscardablePixelRef.cpp
+++ b/src/lazy/SkDiscardablePixelRef.cpp
@@ -71,7 +71,7 @@ bool SkDiscardablePixelRef::onNewLockPixels(LockRec* rec) {
         return false;
     }
 
-    // Note: our ctable is not purgable, as it is not stored in the discardablememory block.
+    // Note: our ctable is not purgeable, as it is not stored in the discardablememory block.
     // This is because SkColorTable is refcntable, and therefore our caller could hold onto it
     // beyond the scope of a lock/unlock. If we change the API/lifecycle for SkColorTable, we
     // could move it into the block, but then again perhaps it is small enough that this doesn't