tour: added missing word "methods"

Missing word "methods" in fourth paragraph of screen "Choosing a value or pointer receiver".  Reads more clearly when what is being talked about is identified.

Fixes golang/tour#1352

Change-Id: I569f7ceed2e732c9e0485270828dcab387f86c71
GitHub-Last-Rev: b086ba8212
GitHub-Pull-Request: golang/website#197
Reviewed-on: https://go-review.googlesource.com/c/website/+/471315
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

_content/tour/methods.article

@@ -131,7 +131,7 @@ The first is so that the method can modify the value that its receiver points to
 The second is to avoid copying the value on each method call.
 This can be more efficient if the receiver is a large struct, for example.
 
-In this example, both `Scale` and `Abs` are with receiver type `*Vertex`,
+In this example, both `Scale` and `Abs` are methods with receiver type `*Vertex`,
 even though the `Abs` method needn't modify its receiver.
 
 In general, all methods on a given type should have either value or pointer