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

@@ -6,8 +6,8 @@ Given the name of a (signed or unsigned) integer type T that has constants
 defined, jsonenums will create a new self-contained Go source file implementing
 
 ```
-    func (t T) MarshalJSON() ([]byte, error)
-    func (t *T) UnmarshalJSON([]byte) error
+func (t T) MarshalJSON() ([]byte, error)
+func (t *T) UnmarshalJSON([]byte) error
 ```
 
 The file is created in the same package and directory as the package that
@@ -18,32 +18,32 @@ most performant or beautiful to read.
 
 For example, given this snippet,
 
-```
-    package painkiller
+```Go
+package painkiller
 
-    type Pill int
+type Pill int
 
-    const (
-        Placebo Pill = iota
-        Aspirin
-        Ibuprofen
-        Paracetamol
-        Acetaminophen = Paracetamol
-    )
+const (
+	Placebo Pill = iota
+	Aspirin
+	Ibuprofen
+	Paracetamol
+	Acetaminophen = Paracetamol
+)
 ```
 
 running this command
 
 ```
-    jsonenums -type=Pill
+jsonenums -type=Pill
 ```
 
 in the same directory will create the file `pill_jsonenums.go`, in package
 `painkiller`, containing a definition of
 
 ```
-    func (r Pill) MarshalJSON() ([]byte, error)
-    func (r *Pill) UnmarshalJSON([]byte) error
+func (r Pill) MarshalJSON() ([]byte, error)
+func (r *Pill) UnmarshalJSON([]byte) error
 ```
 
 `MarshalJSON` will translate the value of a `Pill` constant to the `[]byte`
@@ -58,7 +58,7 @@ change to `Aspirin` and the returned error will be `nil`.
 Typically this process would be run using go generate, like this:
 
 ```
-    //go:generate jsonenums -type=Pill
+//go:generate jsonenums -type=Pill
 ```
 
 If multiple constants have the same value, the lexically first matching name