Update "go install" instructions (ko-build#377)

I got the following error when following the original instructions:

    go install: version is required when current directory is not in a module
            Try 'go install github.com/google/ko@latest' to install the latest version

README.md

@@ -45,7 +45,7 @@ brew install ko
 With Go 1.16+, build and install the latest released version:
 
 ```
-go install github.com/google/ko
+go install github.com/google/ko@latest
 ```
 
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