docs: improve explanation of the default.

Author: ldez

.golangci.example.yml

@@ -66,8 +66,8 @@ run:
 
   # Define the Go version limit.
   # Mainly related to generics support in go1.18.
-  # Default: 1.17
-  go: 1.18
+  # Default: use Go version from the go.mod file, fallback on the env var `GOVERSION`, fallback on 1.17
+  go: '1.18'
 
 
 # output configuration options

pkg/config/config.go

@@ -57,7 +57,7 @@ func IsGreaterThanOrEqualGo118(v string) bool {
 func DetectGoVersion() string {
 	file, _ := gomoddirectives.GetModuleFile()
 
-	if file != nil && file.Go != nil {
+	if file != nil && file.Go != nil && file.Go.Version != "" {
 		return file.Go.Version
 	}