spec: add missing space in EBNF

Fixes #36520

Change-Id: I698ab235f82f7c81caa09318c954847cf3833153
GitHub-Last-Rev: 368a1dc7889c2370fba272bcb45d94822b60d7b9
GitHub-Pull-Request: golang/go#36559
Reviewed-on: https://go-review.googlesource.com/c/go/+/214821Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Jan 13, 2020",
+	"Subtitle": "Version of Jan 14, 2020",
 	"Path": "/ref/spec"
 }-->
 
@@ -2413,7 +2413,7 @@ operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
 Operand     = Literal | OperandName | "(" Expression ")" .
 Literal     = BasicLit | CompositeLit | FunctionLit .
 BasicLit    = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
-OperandName = identifier | QualifiedIdent.
+OperandName = identifier | QualifiedIdent .
 </pre>
 
 <h3 id="Qualified_identifiers">Qualified identifiers</h3>