bump go version to 1.15 in docs

shmokmt · shmokmt · commit bcc1b433ab29 · 2020-08-14T10:00:04.000+09:00
diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
@@ -23,7 +23,7 @@ Please avoid:
 ## Building the project
 
 Prerequisites:
-- Go 1.14
+- Go 1.15
 
 Build with: `make` or `go build -o bin/gh ./cmd/gh`
 
diff --git a/docs/source.md b/docs/source.md
@@ -1,10 +1,10 @@
 # Installation from source
 
-0. Verify that you have Go 1.13.8+ installed
+0. Verify that you have Go 1.15+ installed
 
    ```sh
    $ go version
-   go version go1.14
+   go version go1.15
    ```
 
    If `go` is not installed, follow instructions on [the Go website](https://golang.org/doc/install).
