chore: regenerate README (#376)

This PR was generated using Autosynth. 🌈


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

```
2021-03-03 19:44:32,823 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-iamcredentials/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-03 19:44:33,688 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/d03a5992-3e0e-4cc1-9a28-06a786482027/targets

- [ ] To automatically regenerate this PR, check this box.

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-iamcredentials.git",
-        "sha": "c14c4226cc5e2361936144c81a0f136b06bd2066"
+        "sha": "4723c289cad405365922a30107426b6bceddb0e0"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8c5628b86cfa8386de7b8fc1675e6b528b552d57"
+        "sha": "2e7bd5cc66a3f0b90ce1c150eb35c89eeb9872d6"
       }
     }
   ]

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>18.0.0</version>
+      <version>18.1.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -45,7 +45,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:18.0.0')
+implementation platform('com.google.cloud:libraries-bom:18.1.0')
 
 compile 'com.google.cloud:google-cloud-iamcredentials'
 ```