[gradle] consistent use of maven url in gradle files (#5045)

* wrap maven url with uri function

* consistent use of maven url in gradle files

docs/plugins.md

@@ -51,7 +51,7 @@ To include in your project, add the following to `build.gradle`:
 buildscript {
   repositories {
     mavenLocal()
-    maven { url = "https://repo1.maven.org/maven2" }
+    maven { url "https://repo1.maven.org/maven2" }
   }
   dependencies {
     classpath "org.openapitools:openapi-generator-gradle-plugin:3.3.4"
@@ -96,4 +96,4 @@ openApiGenerate {
         dateLibrary: "java8"
     ]
 }
-```
+```