runtime: fix comment typo in page allocator

A commit looks to have some minor bug that makes comments look confusing.
The commit in question: https://go-review.googlesource.com/c/go/+/250517

Change-Id: I7859587be15a22f8330d6ad476058f74ca2ca6ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/490795
Run-TryBot: shuang cui <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cuishuang

src/runtime/mpagealloc_64bit.go

@@ -90,7 +90,7 @@ func (p *pageAlloc) sysInit(test bool) {
 // sysGrow performs architecture-dependent operations on heap
 // growth for the page allocator, such as mapping in new memory
 // for summaries. It also updates the length of the slices in
-// [.summary.
+// p.summary.
 //
 // base is the base of the newly-added heap memory and limit is
 // the first address past the end of the newly-added heap memory.