README: Improve formatting by removing unneeded indent.

Since a fenced code block (three backticks) is already used, there's no need to indent code blocks. Either a fenced code block or an indent alone is enough.
Also use tabs instead of spaces in Go code to make it gofmt-compatible, and use Go syntax highlighting for Go code blocks.
Unlike 1e1ffecf4f148c30fb214feeb491c7de5fe54e56, only use Go syntax highlighting for complete Go code block, since GitHub renders it poorly for partial Go code.

README.md

@@ -18,7 +18,7 @@ most performant or beautiful to read.
 
 For example, given this snippet,
 
-```
+```Go
 package painkiller
 
 type Pill int