chore: regenerate README (#419)

This PR was generated using Autosynth. 🌈


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

```
2021-05-25 17:12:22,538 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-billing/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-25 17:12:23,740 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/54394411-5544-4fdd-bf8f-e3142314ff15/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billing.git",
-        "sha": "6f60a3bcc338ad4504b3c274110a84ff50939f83"
+        "sha": "bb5a1b3eafc76e8a62cf86c4d396f157f2416a2a"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4f4b1b9b8d8b52f1e9e4a76165896debce5ab7f1"
+        "sha": "a2de127f90799cd5bace653b1dcc5dd2ca8b19c9"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.4.0</version>
+      <version>20.5.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -38,14 +38,14 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-billing</artifactId>
-  <version>1.1.17</version>
+  <version>1.1.18</version>
 </dependency>
 
 ```
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.4.0')
+implementation platform('com.google.cloud:libraries-bom:20.5.0')
 
 compile 'com.google.cloud:google-cloud-billing'
 ```