chore: regenerate README (googleapis#563)

This PR was generated using Autosynth. 🌈


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

```
2021-05-17 01:54:35,505 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-storage-nio/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-17 01:54:36,748 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/02613d46-9811-46ad-ba4b-b807dbb98934/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-storage-nio.git",
-        "sha": "3c5a88959e57e21ddd2ccf1aee877299381a1f00"
+        "sha": "75162496830d1eed1fc09865e3c7c10ccbefc38a"
       }
     },
     {

README.md

@@ -20,7 +20,7 @@ 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>20.3.0</version>
+      <version>20.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -49,7 +49,7 @@ 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:20.3.0')
+implementation platform('com.google.cloud:libraries-bom:20.4.0')
 
 compile 'com.google.cloud:google-cloud-nio'
 ```