spec: s/char_lit/rune_lit/

The spec talks explicitly about rune literals but the
respective production is still called char_lit for
historic reasons. Updated the two occurences.

Fixes #4602.

R=rsc, iant, r, ken
CC=golang-dev
https://golang.org/cl/7070048

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of January 6, 2013",
+	"Subtitle": "Version of January 7, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -423,7 +423,7 @@ After a backslash, certain single-character escapes represent special values:
 All other sequences starting with a backslash are illegal inside rune literals.
 </p>
 <pre class="ebnf">
-char_lit         = "'" ( unicode_value | byte_value ) "'" .
+rune_lit         = "'" ( unicode_value | byte_value ) "'" .
 unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
 byte_value       = octal_byte_value | hex_byte_value .
 octal_byte_value = `\` octal_digit octal_digit octal_digit .
@@ -2063,7 +2063,7 @@ or a parenthesized expression.
 <pre class="ebnf">
 Operand    = Literal | OperandName | MethodExpr | "(" Expression ")" .
 Literal    = BasicLit | CompositeLit | FunctionLit .
-BasicLit   = int_lit | float_lit | imaginary_lit | char_lit | string_lit .
+BasicLit   = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
 OperandName = identifier | QualifiedIdent.
 </pre>