fix typo (#10696)

Update doc comment

Signed-off-by: gemaizi <864321211@qq.com>

tsdb/block.go

@@ -40,8 +40,8 @@ import (
 // IndexWriter serializes the index for a block of series data.
 // The methods must be called in the order they are specified in.
 type IndexWriter interface {
-	// AddSymbols registers all string symbols that are encountered in series
-	// and other indices. Symbols must be added in sorted order.
+	// AddSymbol registers a single symbol.
+	// Symbols must be registered in sorted order.
 	AddSymbol(sym string) error
 
 	// AddSeries populates the index writer with a series and its offsets