Update installation command in readme

[vpavic]

While attempting to install memory calculator with the recent version of Go (specifically 1.20.2) I was greeted with the following:

$ go get -v github.com/cloudfoundry/java-buildpack-memory-calculator
go: go.mod file not found in current directory or any parent directory.
	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

This PR updates the install command according to the guidance from the link in the error message from above:

$ go install github.com/cloudfoundry/java-buildpack-memory-calculator@latest
go: downloading github.com/cloudfoundry/java-buildpack-memory-calculator v0.0.0-20190501234420-5148668b0710
go: downloading github.com/spf13/pflag v1.0.3

[linux-foundation-easycla]

• ❌ - login: @vpavic / name: Vedran Pavic . The commit (53f9aef) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[vpavic]

Anything preventing this from getting review and hopefully merged?

If it's the CLA, IMO this is a trivial change (and one that does not touch the code) that would fall into the obvious fix category under the Pivotal CLA. Note that similar changes have been merged before without the CLA.