blog/slices.md: fix minor punctuation typo

Change-Id: I5d12653af8f377ff20d1c8f955634e9cb9c01ea2
GitHub-Last-Rev: 5660e19d3d
GitHub-Pull-Request: golang/website#268
Reviewed-on: https://go-review.googlesource.com/c/website/+/553955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

_content/blog/slices.md

@@ -218,7 +218,7 @@ since the function is passed a copy of the slice header, not the original.
 Thus if we want to write a function that modifies the header, we must return it as a result
 parameter, just as we have done here.
 The `slice` variable is unchanged but the returned value has the new length,
-which is then stored in `newSlice`,
+which is then stored in `newSlice`.
 
 ## Pointers to slices: Method receivers