content: use singular "interface" when discussing Reader

Change-Id: I830a3bf4cfffe96f90c68e725d3806114543f360
GitHub-Last-Rev: dcefcf469c789f262d8452f6a642aa31d235f172
GitHub-Pull-Request: golang/tour#878
Reviewed-on: https://go-review.googlesource.com/c/tour/+/212817
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

content/methods.article

diff --git a/content/methods.article b/content/methods.article
index ab1d40d..34aa673 100644
--- a/content/methods.article
+++ b/content/methods.article
@@ -343,7 +343,7 @@
 The `io` package specifies the `io.Reader` interface,
 which represents the read end of a stream of data.
 
-The Go standard library contains [[https://golang.org/search?q=Read#Global][many implementations]] of these interfaces, including files, network connections, compressors, ciphers, and others.
+The Go standard library contains [[https://golang.org/search?q=Read#Global][many implementations]] of this interface, including files, network connections, compressors, ciphers, and others.
 
 The `io.Reader` interface has a `Read` method: