chore: regenerate README (googleapis#609)

This PR was generated using Autosynth. 🌈


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

```
2021-07-06 17:28:41,082 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-storage-nio/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-06 17:28:42,268 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/2737133a-d2ba-4df3-8ff2-8bb7ad027ef3/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-storage-nio.git",
-        "sha": "def46efd2bfae3605e4dd4eb20ac32b25da6c051"
+        "sha": "ab18d7a8c1c1f8e1b4fd6ce36befc232f48bac6d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "9763f20e4b7bb1091082462b2f7970e965d0d414"
+        "sha": "39652e3948f455fd0b77535a0145eeec561a3706"
       }
     }
   ]

README.md

@@ -42,7 +42,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-nio</artifactId>
-  <version>0.123.2</version>
+  <version>0.123.3</version>
 </dependency>
 
 ```
@@ -55,12 +55,12 @@ compile 'com.google.cloud:google-cloud-nio'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-nio:0.123.2'
+compile 'com.google.cloud:google-cloud-nio:0.123.3'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-nio" % "0.123.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-nio" % "0.123.3"
 ```
 
 ## Authentication