Use raw strings to avoid double escapes (gosimple)

This fixes:
pkg/services/alerting/rule.go:58:21: should use raw string (`...`) with regexp.MustCompile to avoid having to escape twice (S1007\)
pkg/services/alerting/rule.go:59:21: should use raw string (`...`) with regexp.MustCompile to avoid having to escape twice (S1007)

pkg/services/alerting/rule.go

@@ -55,8 +55,8 @@ func (e ValidationError) Error() string {
 }
 
 var (
-	ValueFormatRegex = regexp.MustCompile("^\\d+")
-	UnitFormatRegex  = regexp.MustCompile("\\w{1}$")
+	ValueFormatRegex = regexp.MustCompile(`^\d+`)
+	UnitFormatRegex  = regexp.MustCompile(`\w{1}$`)
 )
 
 var unitMultiplier = map[string]int{