chore: regenerate README (#423)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-06-02 20:17:34,742 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-billing/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-02 20:17:35,975 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/6f2ce86d-3a80-4ec9-abe2-5ea90b5787eb/targets

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billing.git",
-        "sha": "bb5a1b3eafc76e8a62cf86c4d396f157f2416a2a"
+        "sha": "0525b9ee87e9adba9744bbd678106e69ee5f680e"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a2de127f90799cd5bace653b1dcc5dd2ca8b19c9"
+        "sha": "c3e41da0fa256ad7f6b4bc76b9d069dedecdfef4"
       }
     }
   ]

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-billing</artifactId>
-  <version>1.1.18</version>
+  <version>1.2.0</version>
 </dependency>
 
 ```
@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-billing'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-billing:1.1.18'
+compile 'com.google.cloud:google-cloud-billing:1.2.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-billing" % "1.1.18"
+libraryDependencies += "com.google.cloud" % "google-cloud-billing" % "1.2.0"
 ```
 
 ## Authentication