Fix install instructions

[bitdivine]

Motivation

go get is now deprecated for installing executables. See: https://go.dev/doc/go-get-install-deprecation

And indeed doesn't work any more.

Changes

• Replace go get with the replacement go install

Tests

Before:

max@sinkpad:~ (24)$ go get github.com/ericchiang/pup
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'.
max@sinkpad:~ (4:47)$ echo $?
1

After:

max@sinkpad:~ (43)$ go install github.com/ericchiang/pup@latest
go: finding module for package golang.org/x/text/transform
go: finding module for package golang.org/x/net/html/charset
go: finding module for package golang.org/x/net/html
go: finding module for package github.com/fatih/color
go: finding module for package golang.org/x/net/html/atom
go: finding module for package github.com/mattn/go-colorable
go: found github.com/fatih/color in github.com/fatih/color v1.15.0
go: found github.com/mattn/go-colorable in github.com/mattn/go-colorable v0.1.13
go: found golang.org/x/net/html in golang.org/x/net v0.8.0
go: found golang.org/x/net/html/atom in golang.org/x/net v0.8.0
go: found golang.org/x/net/html/charset in golang.org/x/net v0.8.0
go: found golang.org/x/text/transform in golang.org/x/text v0.8.0
max@sinkpad:~ (2:24)$ 
max@sinkpad:~ (5:13)$ echo $?
0
max@sinkpad:~ (5:15)$ which pup
/home/max/gopath/bin/pup
max@sinkpad:~ (5:24)$ 

[gromgit]

Pretty much a duplicate of #181 and #185, and since the project lead hasn't responded to any PRs in six years, this PR probably won't be merged either.