Update Go version in README.md (#5471)

Prometheus 2.9 requires Go 1.12+ to build it from source
This commit updates the README file according to this requirement
Signed-off-by: Vandewilly Silva <vandewilly.oli.silva@hpe.com>

README.md

@@ -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`: