Update comment to reflect correct constant. (#1144)

The constant is `DebugMode`, as defined in [mode.go](https://github.com/gin-gonic/gin/blob/1e88466d234a82ce4aeca904bce8a0b93fac3d42/mode.go#L16).

debug.go

@@ -15,7 +15,7 @@ func init() {
 }
 
 // IsDebugging returns true if the framework is running in debug mode.
-// Use SetMode(gin.Release) to disable debug mode.
+// Use SetMode(gin.ReleaseMode) to disable debug mode.
 func IsDebugging() bool {
 	return ginMode == debugCode
 }