BUILD.md: Update Go version requirement (#2332)

    src/options.go:463:9: undefined: strings.ReplaceAll
    note: module requires Go 1.13make: *** [Makefile:122: target/fzf-linux_amd64] Erro 2

BUILD.md

@@ -6,7 +6,7 @@ Build instructions
 
 ### Prerequisites
 
-- Go 1.11 or above
+- Go 1.13 or above
 
 ### Using Makefile