Update Go version in README.md
Prometheus 2.9 requires Go 1.12+ to build it from source
This commit updates the README file according to this requirement
@@ -58,7 +58,7 @@ Prometheus will now be reachable at http://localhost:9090/.
### Building from source
To build Prometheus from the source code yourself you need to have a working
-Go environment with [version 1.11 or greater installed](https://golang.org/doc/install).
+Go environment with [version 1.12 or greater installed](https://golang.org/doc/install).
You can directly use the `go` tool to download and install the `prometheus`
and `promtool` binaries into your `GOPATH`:
