fix a typo in the runtime.MemStats documentation

Change-Id: If553950446158cee486006ba85c3663b986008a6
Reviewed-on: https://go-review.googlesource.com/82936
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: pboyd

src/runtime/mstats.go

@@ -262,7 +262,7 @@ type MemStats struct {
 	// can only be used for other objects of roughly the same
 	// size.
 	//
-	// HeapInuse minus HeapAlloc esimates the amount of memory
+	// HeapInuse minus HeapAlloc estimates the amount of memory
 	// that has been dedicated to particular size classes, but is
 	// not currently being used. This is an upper bound on
 	// fragmentation, but in general this memory can be reused